How to Sleep well at night?
– Dr Tripat Deep Singh

A lot of sleep disorders can disturb your sleep at night. Insomnia is one of the sleep disorders and is defined as trouble falling asleep or staying asleep at night. There are different factors that can contribute to Insomnia including bad sleep habits, consuming substances or having a bad sleeping environment that is not conducive for falling or maintaining Sleep.

When I see a patient, who complains of difficulty falling asleep or maintain sleep at night, I try to determine the underlying cause for the patient’s problem. Detailed medical and sleep history is taken which allows me to determine what might be causing the problem.

Sleep hygiene include behaviors around sleep timing, pre-bed routines and sleeping environment. I enquire in detail about patient’s behaviors regarding the sleep wake schedule on weekdays and weekends and napping habits, regarding patient’s pre-bed routines including timing of dinner and exercise and about sleeping environment including temperature, humidity, light, sound, mattress, pillows. Any of these factors can be the cause or perpetuating factor for Inadequate sleep hygiene Insomnia.

Addressing inadequate sleep hygiene may not cure other types of Insomnia not due to inadequate sleep hygiene but may help to augment the clinical gains of other treatments and help to prevent recurrence.

This makes it very important for me to identify the underlying cause of Insomnia, have a detailed discussion with patient regarding their Sleep hygiene habits, identify the bad sleep hygiene habits and explain to them the rationale for each advice regarding improving their sleep hygiene habits.

In today’s Google world, many patients have been exposed to the list of good Sleep hygiene habits on the internet and many have tried practicing them on their own. But I feel more important than the list is patients understanding of the sleep wake regulation and the science behind each of the Sleep hygiene tips for them to adhere to the treatment and benefit from it. I will share an example here of one of my patients who had problem falling asleep at night. This patient was trying to have increased deep sleep at night as compared to REM. He was monitoring his sleep with a smart watch and every morning will look whether deep sleep have increased or not with his different sleeping tricks at night. This patient had no clear understanding regarding how sleep is distributed throughout the night and no need to increase particular sleep as long as it is within normal duration. Explanations around the types of sleep, distribution and importance of sleep at night helped the patient to stop worrying about increasing the deep sleep and helped him to have easy sleep onset as his mind was relaxed before going to sleep.

I am sharing with you good Sleep hygiene tips for you to have better night Sleep but I will suggest that if you have trouble falling or maintaining sleep and you have either good or poor Sleep hygiene, consult a Sleep physician for the right cause to be identified underlying the Insomnia before we try only good Sleep hygiene tips without any improvement and labelling them as a failure to cure your Insomnia problem. Identifying the right cause, instituting the right treatment along with good Sleep hygiene can do wonders to cure your Insomnia problem. There may be another underlying sleep disorder other than Insomnia contributing to your sleep problems. Sleep Physician will be able to assess the right cause and start the right treatment for your Sleep disorder.

Sleep Hygiene tips for Adults

Sleep hygiene tips are simple recommendations that may influence length and improve the quality of sleep by modification of habits, behaviors and environmental conditions. There is a lot of clinical evidence regarding good sleep hygiene and improved Sleep and performance.

  • Bad Sleep hygiene practices like using nicotine late in the evening, light or noise disturbance, and having an irregular sleep schedule have been shown to be associated with concurrent and future Insomnia.1
  • In athletes with poor sleep, following Sleep hygiene advice has been shown to improve Sleep and enhance athletic performance.2 Banking sleep (sleep extension prior to night of intentional sleep deprivation before sporting event) is a new concept that may also improve performance.2
  • Sleep hygiene principles have been shown to benefit postmenopausal women who suffer from insomnia.3
  • Combined light exposure and sleep hygiene improved speed and power but not intermittent-sprint performance up to 96 hours following long-haul transmeridian travel.4
  • Sleep hygiene maintenance helps persons living with diabetes enjoy a better quality of life by restoring normal sleep patterns and thereby improving glycaemic control.5

I will be sharing 20 tips to improve Sleep hygiene for better night Sleep under following categories-

  • Behaviour change
  • Healthy pre-sleep routine
  • Improving bedroom environment

Behaviour change

Tip1: Have a regular Sleep-wake schedule on weekdays and weekends

Having a consistent sleep wake schedule on weekdays and weekends helps you to keep your circadian system aligned to the light and dark cycle. Circadian system not only controls your Sleep- Wake cycle but also controls your metabolic functions, temperature rhythms, appetite and hormone rhythms.

Another issue with not maintaining a regular sleep-wake schedule on weekdays and weekends is that one tend to accumulate “Sleep debt”. Now-a-days people sleep less on weekdays due to work pressure or lifestyle changes and try to sleep more on weekends to make up for lost Sleep. When someone is sleeping less than what is required, they accumulate “Sleep debt” eg: if someone needs 7hrs Sleep per night and sleep only 6hrs, then the person is accumulating 1hr of Sleep debt per day. Sleep debt is additive, if you are sleeping 1 hr less every night then sleep debt gets accumulated over the week.

Sleep debt has been defined as difference >60min between total sleep time required for the person and actual sleep hours obtained on weekdays. Severe sleep debt is difference >90min between total sleep time required for the person and actual sleep hours obtained on weekdays.6

I usually suggest my patients that having a regular sleep-wake schedule on weekdays and weekends helps you to avoid accumulating Sleep debt in the first place along with its harmful consequences on the health.

One can follow following tips to achieve regular sleep-wake schedule and avoid sleep debt-

  1. Determine your Sleep need: Along with your Physician try to understand how much Sleep is required by you. Each individuals Sleep requirements are different. Age specific guidelines on Sleep requirements have been published by National Sleep Foundation which can be referred for recommendations.
  2. Set your sleep and wake up time: Set you Sleep and wake up time according to the number of hours of Sleep required by you as determined along with your physician. Smart devices now-a-days can help you to set reminders for your Sleep time 1 hr before your scheduled time for Sleeping so that you can start winding down and get ready for sleep on time.
  3. Avoid Sleep disturbing activities during daytime: Avoid any daytime activities that may interfere with night time Sleep like drinking caffeine 4-6hrs before sleep time, consuming alcohol, having heavy dinner or exercising too close to bedtime.
  4. Comfortable bedroom environment: Try to have a comfortable bedroom environment with right temperature, humidity and sound levels, a comfortable mattress, pillows or linen which helps you to sleep better. Please read tips on improving bedroom environment in this blog for more specific details on how to achieve a comfortable bedroom environment.
  5. Stop doing work 1 hr prior to your sleep time: Start preparing yourself for Sleep at least 1 hr prior to your scheduled sleep time. Read more tips about changing your pre-sleep routine in this blog.

If you have already accumulated Sleep debt, then in addition to above tips for maintaining regular sleep-wake schedule you can follow following tips to pay your sleep debt-

  1. Determine your Sleep need: Along with your Physician try to understand how much Sleep is required by you.
  • Give yourself sometime: Increase your Sleep at night in increments of 15-30min every night until you reach your goal of sleep duration as determined along with your Physician. It may take some time to pay your sleep debt depending upon how much Sleep debt has been accumulated and if you do not continue to accumulate sleep debt.
  • Napping and weekend catch up sleep: The practice of sleeping more on weekend to make up for lost sleep on weekdays has been referred to as “Weekend Catch up Sleep”.One can try napping and weekend catch up Sleep to pay your sleep debt but these two should not be your primary methods to pay your sleep debt.

While sometimes life’s situations make us to lose sleep on weekdays and catching up on Sleep and napping over the weekend may make us feel better but it should not become a routine once the situations improve. Accumulating sleep debt on weekdays and sleeping longer on weekends may give you a false sense of well-being but the Sleep debt snow balls over a period of time.

Studies have shown that napping and weekend catch up sleep compensated for severe sleep debt only in 1 in 4 cases only.6 Also studies have shown that if you accumulate 1 hr of sleep debt it may take 4 nights of adequate Sleep to pay that debt.7

If you are feeling sleepy during daytime despite sleeping as per your body needs and not accumulating sleep debt, please consult a Sleep Physician to screen for any underlying Sleep disorder.

Tip2: Avoid Naps 6-8hrs before Sleep time

Two processes regulate Sleep- Homeostatic process (Process S) and circadian process (Process C). Process S is the sleep pressure that builds up during the day while you are awake. Longer you are awake, higher the sleep pressure.

Napping during the daytime takes off the sleep pressure and makes it difficult to fall asleep during the night time.

Some of my patients especially retirees tell me that they have free time in the afternoon to take a nap and they feel fresh after that. I usually tell them to avoid naps as occasional naps may lead to regular napping via conditioning and this may alter the sleep wake timing and affect sleep homeostasis.

My advice to my patients is nap only if necessary to maintain alertness. If it is necessary to take a nap then try to keep the duration of the nap to 10-20min only8 and try to have the nap in the morning or early afternoon as it will not interfere with night time Sleep. Taking a nap after 3pm will definitely interfere with your night time Sleep as it will reduce the Sleep pressure built up during the day. My general advice to my patients is to avoid naps after 3pm or 6-8hrs before regular sleep time to avoid altering Sleep homeostasis.

Some patients complain that if they take a nap for 1-2hrs in the afternoon they wake up groggy rather than fresh. This feeling of grogginess after a long nap is referred to as “Sleep Inertia”.8 This happens as long nap allows you to enter Stage N3 Sleep and you feel groggy when you wake up from stage N3 sleep. Keeping the nap short to 10-20min duration does not give you enough time to enter Stage N3 sleep and you wake up fresh from the short nap.

If you are sleepy during daytime despite having sleep duration as per your body needs as determined along with your Sleep physician and despite having naps, there may be an underlying Sleep disorder for which you may need Sleep Physician consultation.

Healthy pre-sleep routine

Tip3: Try relaxing 1hr prior to your Sleep time

At night levels of melatonin hormone, secreted from Pituitary gland, starts increasing. Melatonin hormone promotes Sleep. On the other hand, levels of Cortisol hormone decrease throughout the night in sleeping period. Cortisol is the stress hormone and its levels starts increasing just before waking up to get us ready for the stresses of the daily life. At night if we are stressed, it activates stress response which causes release of cortisol and increased cortisol levels promotes wakefulness.

Relaxing before sleep avoids stress response from being activated and releasing cortisol which cause wakefulness.

One can try any of the following techniques to relax- Mindfulness meditation, guided imagery, hypnosis, progressive muscle relaxation, and deep breathing.

One can try Techniques with physical component such as yoga, tai-chi and qi gong.9

If worrying thoughts about next day or future trouble you and you are unable to fall asleep, my personal advice is try writing these worrying thoughts in a diary which has helped a lot of my patients to fall asleep. Write down specific to-do-list for 5 minutes rather than chronicling all completed activities. It has been shown in a study that participants who wrote to-do-list fell asleep faster than participants who just wrote about completed activities.10

Take 5min to be thankful for all the blessings of your life, lie down comfortably, close your eyes, breathe slowly and being thankful let the Sleep embrace you.

Tip4: Take Hot shower 1-2hrs before Sleep time for 10min

Core Body temperature have circadian rhythm with peak in late afternoon and early evening and falls at the onset of Sleep. One sleeps on falling temperature curve of the body.

Taking hot shower increases the temperature of body extremities which is detected by heat sensing centres in the brain. Heat sensing centres detect increased temperature of the extremities and activate the cold response which dilates the blood vessels in extremities leading to heat loss which drops the body core temperature. Falling of body core temperature helps one to fall asleep easily.11

I tell my patients not to take a hot shower immediately before going to bed as it will raise the core body temperature and make it difficult to fall asleep. A hot shower should be taken 1-2hrs prior to your scheduled sleep time to provide enough time for your core body temperature to fall which will assist with sleep onset.

Tip5: Do Regular exercise but 90min before Sleep time

Core Body temperature have circadian rhythm with peak in late afternoon and early evening and falls at the onset of Sleep. One sleeps on falling temperature curve of the body.

Exercising too close to bed time will increase the core body temperature which will make falling asleep difficult.

I advise my patients to exercise at least 90min before Sleep time.  This will increase core body temperature also but provides some time for the core body temperature to fall by the time one goes to Sleep.12 You can do following to set your exercise routine-

  1. Determine your Sleep need: Along with your Sleep Physician try to understand how much Sleep is required by you. Each individuals Sleep requirements are different.
  2. Set your sleep and wake up time: Set your Sleep and wake up time according to the number of hours of Sleep required by you as determined along with your physician.
  3. Set an exercise time at least 90min away from your set sleep time. You can set a reminder for set exercise time on your smart devices.

Tip6: Avoid heavy meals 2-3hrs before Sleep time

Having dinner 2-3hrs before bedtime decrease number of nocturnal awakenings, makes it easy to fall asleep and increases total sleep time in healthy young adults.13

There are circadian rhythm system in your body which controls your sleep-wake cycle, metabolism, hormone levels  and many other bodily functions. The master clock to set the rhythms is a small structure in your brain called “Suprachiasmatic nucleus”. Each cell in your body have its own clock (Peripheral clocks) which is in tune with your master clock if you have healthy circadian system. Light, food, sleep and wakefulness helps to entrain our circadian system. Master clock have influence on peripheral clocks which in turn can influence the master clock.

If you eat a heavy meal very close to your sleep time, food may reset your peripheral clocks in liver and other gut organs to digest food and this may cause peripheral clocks not to be in tune with your master clock. In simple terms, master clock may be putting you to sleep but your peripheral clocks are keeping the organs active which have its own health consequences.

I advise my patients to have a light meal at least 2-3hrs before their sleep time to maintain synchrony between their master and peripheral clocks. Eating a light meal 2-3hrs before sleep time gives enough time for digestion to happen and maintain healthy circadian system and good quality Sleep.

Avoid eating spicy foods, red meat and carbonated beverages at night. Choose a light carbohydrate rich diet at night.

You can start with setting a reminder to eat 2-3hrs before sleep time until it becomes a habit.

Tip7: Listen to relaxing music

Listening to relaxing music especially including naturalistic sounds helps to feel pleasant and decrease distractions.14

Choose anything in classical music or natures sounds to relax you. You will need to try  couple of music pieces to see which ones relax you the most. Like in my case after a lot of listening, I have chosen Indian classical ragas which are specific to night time like Raga JaiJaiwanti, Raga Des, Raga Darbari as they help me to relax and go to sleep in a very relaxed manner.

One can do following-

  1. Dim the lights 1hr before your sleep time,
  2. Keep the gadgets away
  3. Switch on your chosen piece of music
  4. If you are playing music on your phone/tablet or PC, make sure to switch off all the notifications for this one hour and possibly for the night.

Tip8: Allow 3-4hrs between drinking alcohol and Sleep time

Alcohol intake decreases time to fall asleep. But it disturbs Sleep architecture in several ways which make you wake up not feeling refreshed. Alcohol intake close to sleep time have the following effects15

  • The onset of the first REM sleep period is significantly delayed
  •  Reduction in total night REM sleep.
  • Increase in slow wave sleep (SWS) in the first half of the night relative to baseline values
  • Aggravates OSA
  • Cause more urination and disrupt sleep
  • Increase risk of Parasomnias

My advice to you regarding drinking is as follows-

  • Avoid drinking before sleep time
  • If you have to drink then Allow 3-4hrs between drinking and Sleep time
  • Stay hydrated and drink water
  • Avoid bubbly drinks as they cause bloating and gas
  • Eat a light snack before bedtime

Tip9: Do not consume a lot of caffeine 4-6hrs before Sleep time

Two processes regulate Sleep- Homeostatic process (Process S) and circadian process (Process C). Process S is the sleep pressure that builds up during the day while you are awake. Longer you are awake, higher the sleep pressure.

The neurotransmitter for Process-S is Adenosine. Adenosine levels in your brain increase with increasing wakefulness and makes us feel sleepy. Caffeine blocks adenosine receptors and hence adenosine action producing state of alertness/wakefulness.

400 mg of caffeine taken 0, 3, or even 6 hours prior to bedtime significantly disrupts sleep.

Even at 6 hours, caffeine reduced sleep by more than 1 hour.16

Even a single cup of caffeinated coffee (90mg) consumed before bedtime in real-life conditions causes a deterioration in the quality of sleep in caffeine-sensitive subjects.17

Just say no to any caffeinated or energy drink after 3pm if you are sensitive to caffeine.

Tip10: Drink a warm glass of milk or a light snack  before bedtime18

Drinking a warm glass of milk or a light snack may help to prevent fall of blood glucose levels at night which can disturb sleep.

Carbohydrates like crackers, bread, cereals and fruits are best for a good night Sleep. Try to avoid chocolates or heavily sweetened foods as snacks.

Please talk to your Physician If you are intolerant to lactose or milk.

Tip11: Do not smoke 4-6hrs before Sleep time

Nicotine is a stimulant and promotes wakefulness. Late evening use of nicotine has been shown to cause Insomnia and increases the risk to develop Insomnia at 1yr.1

Try following if you are a chronic smoker with difficulty sleeping-

  1. Try to enroll in “quit smoking” program
  2. Try nicotine patch
  3. Avoid smoking in the middle of the night if you wake up and cannot go back to sleep
  4. Try changing the timing of smoking 4-6hrs away from bedtime and see if it helps

Improving Bedroom Environment

Tip12: Ensure bedroom is only used for sleep or intimacy

The scientific principle for this advice is to develop a Conditioning response to identify the bed & the bedroom with sleep & not a place for other activities.

My advice regarding using bedroom only for sleep or intimacy is-

  1. Take out the television out of your bedroom,
  2. Keep all gadgets out of the bedroom for the night
  3. Keep an ordinary clock in your room and not a digital clock or any smart watch or gadgets be used as watch
  4. Do not play games or do household chores in your bed

Tip 13: Bedroom environment should be quiet

Some people are not easily awakened by slight noises at night but some are very sensitive to even the slightest of the sound which will wake them up. People who get easily disturbed by noise should try to reduce noise levels or acoustically isolate the room which helps to increase deep sleep.19

One can use ear plugs also to block noise. They reduce noise levels by 30-35 dB.19 White noise can be used to improve sleep quality if your sleep is disturbed by environmental noise.20 But if you are caregiver, please do not use ear plugs as you may not be able to hear the call when called for any help or support.

Tip14: Maintain ambient bedroom temperature and humidity

Different people prefer different room temperature and humidity levels comfortable enough for Sleeping. I am sharing some scientific data that has been published with regards to room temperature and humidity levels for a comfortable sleep, which may be useful to some of you-

  • Try to Maintain room temperature of 27-30 degree Celsius21 with a humidity of 50-70%22 for good Sleep. In daily life, the use of air conditioning in the initial sleep hours can protect sleep and thermoregulation.22
  • Another approach that has been suggested for refreshing awakening is to gradually decrease room temperature from 27 degrees C to 24 degrees C between 1800 and 2200 h and gradual increase from 24 degrees C to 27 degrees C between 0300 and 0700 h promoted sleep and fresh awakening and increased melatonin levels at night.23

Tip15: Try to have comfortable bedding accessories

The choice of bedding accessories (comfortable pillow, quilt, bed sheets, night clothing) for good night sleep is also very personal and can vary from person to person. I am sharing some scientific data that has been published with regards to choosing bedding accessories for a comfortable sleep, which may be useful to some of you-

  • Comfortable pillows made of special material help reduce core body temperature and improve Sleep24
  • Proper choice of quilts also decrease core body temperature better and improve Sleep25
  • Use of electric blanket in winters can help to improve Sleep stability and thermoregulation during Sleep26
  • Wearing socks at 23 degree Celsius room temperature with 50% relative humidity improves Sleep quality27

I usually advice my patients to change their pillows at least once in every 1-3years or earlier if they wear out.

Tip16: Choose a comfortable mattress

Choice of mattress for good night sleep is again a very personal choice. I will share some scientific data on which mattress features have been shown to have a positive effect on Sleep which may allow some of you to make an informed decision while choosing your own mattress for a good night Sleep-

  • Prefer a mattress which provides head and back cooling28
  • A mattress with high rebound topper provides more restorative Sleep29
  • Prefer a mattress with high heat capacity as it improves deep sleep30
  • Medium firm or firm mattress may benefit back pain31
  • Prefer a mattress which provides support to your shoulders and hip32
  • If you have Positional OSA, sleeping on a specially designed pillow and mattress reduces severity of OSA and improves oxygenation33
  • If you are prone to back pain, avoid sleeping on a foam mattress as it can lead to back pain34

I do enquire about when was the last time they changed their mattress as sleeping on worn out mattress can disturb sleep. I usually advice my patients to change their mattress at least once in a decade or earlier if mattress is worn out and disturbing their Sleep.

Tip17: Keep bedroom dark and screen free

Hormone Melatonin is released from Pineal gland with onset of darkness and melatonin helps with sleep onset. Melatonin levels are highest during Sleep and its secretion is inhibited with daylight especially the blue wavelength.

Reducing evening blue light advances sleep and melatonin.35 Use of Blue-light-blocking lenses (89-99,9% filter of blue light between 400-500 nm, general decrease of light intensity: 50%) 9hrs before switching off the lights advanced sleep and melatonin levels.35

Use of any type of screen- smart phone, tablet or computer- suppresses melatonin secretion and delays sleep onset because all these devices emit light with blue wavelength which suppresses melatonin secretion.36

Avoid reading ebooks on tablets at night as it has been shown that reading a story from an iPad decreased subjective sleepiness, delayed the EEG dynamics of slow wave activity by approximately 30 min.37

My personal advice to all of you is as follows-

  • Turn off overhead lights two hours before bed, and switching to dimmer table lamps
  • And if you do use a computer or smartphone in the evening, install an app that automatically dims the screen and filters out the blue light
  • Keep all electronic gadgets out of your bedroom
  • Do not check time on your smart phone if you wake up in the middle of the night. Try to go back to sleep without worrying about the time of the night.
  • Try to have heavy curtains cover the window to cut of any unwanted light coming into your bedrooms.

Tip18: Use Aromatherapy (Lavender and Peppermint)

Choice of the aroma to relax oneself can vary from person to person. I will share some of the scientific data showing that some particular fragrances helped to improve Sleep in specific patient populations and we all can try these fragrances to see if we derive the same benefits as highlighted in these studies-

  • Lavender essential oil increased quality of sleep and reduced level of anxiety in patients with coronary artery disease38
  • Peppermint and Lavender oil aromatherapy improved sleep quality in cancer patients39
  • Application of Lavender cream improved Sleep quality in pregnant ladies40
  • Lavender and sleep hygiene together, and sleep hygiene alone to a lesser degree, improved sleep quality for college students with self-reported sleep issues41

Tip19: Keep room air Clean

If room air is not clean, it may irritate your air passages leading to sneezing, coughing or asthma attack disturbing your sleep.

One can try following to keep room air clean-

  • Avoid smoking inside room. It increases levels of toxic chemicals in room air as compared to non-smokers room42
  • Maintain good airflow using either ceiling fan or air conditioner43
  • If you have OSA, indoor air pollution can aggravate severity of OSA which can disturb your Sleep44
  • Avoid sleeping with pets in the room if you are allergic to their hair

Try using air purifier if air quality is really bad in your area.

Tip20: Limit the time you spend awake in bed

The idea behind this advice is scientific principle of “Paradoxical Intention” which states that the harder one tries to sleep farther away it gets.

If awake in bed for 15-20 minutes, get up & leave bedroom. Engage in quiet activity like reading a book, listening to relaxing music or meditation until drowsy again.

Do not engage yourself to watch movies, listen to disturbing music or switch on the bright lights. If you need to navigate the room, try to have a dim light by your bedside.

I hope these tips help you to Sleep better at night. In my next blog I will be discussing about harmful effects of sleep deprivation on our health. Until then- Sleep well and sleep on time.

References:

1.         Jansson-Fröjmark M, Evander J, Alfonsson S. Are sleep hygiene practices related to the incidence, persistence and remission of insomnia? Findings from a prospective community study. J Behav Med. 2019;42(1):128-138.

2.         Vitale KC, Owens R, Hopkins SR, Malhotra A. Sleep Hygiene for Optimizing Recovery in Athletes: Review and Recommendations. Int J Sports Med. 2019;40(8):535-543.

3.         Duman M, Timur Taşhan S. The effect of sleep hygiene education and relaxation exercises on insomnia among postmenopausal women: A randomized clinical trial. Int J Nurs Pract. 2018;24(4):e12650.

4.         Fowler PM, Knez W, Thornton HR, et al. Sleep Hygiene and Light Exposure Can Improve Performance Following Long-Haul Air Travel. Int J Sports Physiol Perform. 2020;16(4):517-526.

5.         Sousa A, Kalra S. Sleep hygiene and diabetes: Suggestions for primary care. J Pak Med Assoc. 2017;67(5):814-815.

6.         Leger D, Richard JB, Collin O, Sauvet F, Faraut B. Napping and weekend catchup sleep do not fully compensate for high rates of sleep debt and short sleep at a population level (in a representative nationwide sample of 12,637 adults). Sleep Med. 2020;74:278-288.

7.         Kitamura S, Katayose Y, Nakazaki K, et al. Estimating individual optimal sleep duration and potential sleep debt. Sci Rep. 2016;6:35812. 8.         Brooks A, Lack L. A brief afternoon nap following nocturnal sleep restriction: which nap duration is most recupera