Sleep FAQ

It counts in whole sleep cycles. For a bedtime it subtracts the time you take to fall asleep and then 90 minutes per cycle from your wake-up time; for a wake-up time it adds them to your bedtime. Cycles average about 90 minutes [1]. Try the sleep calculator.

Only roughly. Cycles vary: the first is often 70–100 minutes and later ones 90–120 minutes [1]. A calculator can’t know which stage you’re in, so treat the times as a starting point and adjust the settings if they don’t suit you.

About 90 minutes on average, ranging from roughly 70 to 120 minutes [1]. See sleep cycles explained.

Not as a night’s sleep. Adults are advised to get 7 or more hours a night [2]. One full cycle (about 90 minutes) can work as a nap [3].

7.5 hours. Both are whole numbers of 90-minute cycles, but only 7.5 hours meets the 7+ hours recommended for adults [2].

Not regularly, for adults. The AASM and Sleep Research Society link regularly sleeping less than 7 hours with adverse health outcomes, including impaired performance and a greater risk of accidents [2].

Most healthy people fall asleep within 15 to 20 minutes of lying down [4]. Falling asleep within minutes could be a sign of insufficient sleep or a medical condition [4].

With 15 minutes to fall asleep: 8:45 PM (6 cycles, 9 hours) or 10:15 PM (5 cycles, 7.5 hours). 11:45 PM is 4 cycles, below the 7+ hours most adults need. See the 6 AM page for the full table.

Newborns 14–17 hours; 4–12 months 12–16; 1–2 years 11–14; 3–5 years 10–13; 6–12 years 9–12; teens 8–10; adults 7 or more; 65+ 7–8 [5]. See the full chart with sources.

The CDC, AASM and National Sleep Foundation recommendations give one range per age group and don’t list separate figures for men and women [5][2][6].

8 to 10 hours per 24 hours for ages 13 to 18, according to the AASM [7].

Sleep debt is the shortfall between the sleep you need and the sleep you get, added up over several nights. Paying it back isn’t simple: NINDS says sleeping longer at weekends may not be enough to replace missed sleep [8]. Try the sleep debt calculator.

There’s no fixed figure. In one laboratory study, recovering from 1 hour of potential sleep debt took four days [9], and another found that weekend recovery sleep didn’t prevent the metabolic effects of repeated short sleep [10].

For most adults the Sleep Foundation suggests about 20 minutes and no longer than 30; a nap of around an hour and a half may also be beneficial [3]. See the nap calculator.

Experts typically recommend that adults nap eight or more hours before bedtime, which for most people means before 3 p.m. [3].

One study supports avoiding substantial caffeine for at least 6 hours before bed [11]; a 2023 meta-analysis suggests a coffee at least 8.8 hours before bedtime [12]. Use the caffeine cutoff calculator.

It may make you sleepy at first, but it disrupts normal sleep patterns, leading to lighter, less restorative rest and more awakenings, and it reduces REM sleep [13]. CDC suggests avoiding alcohol before bedtime [5].

It can leave you groggy for a while. Deep sleep is the stage in which it may be difficult to awaken you [8], and waking from it can cause grogginess [3].

The grogginess and reduced performance just after waking. It fades the longer you’re awake and is worse after sleep loss and at certain times of day [14].

There are many possible reasons, and we can’t diagnose yours. Regularly waking in the night and lying awake is one of the symptoms of insomnia listed by the NHS [15]. If it keeps happening, or you’re tired during the day, talk to a doctor or GP [5][15].

Start with the habits CDC lists: going to bed and getting up at the same time every day, a quiet, cool bedroom, devices off at least 30 minutes before bed, and no caffeine in the afternoon or evening [5]. Use the bedtime calculator to choose a bedtime you can keep.

The evidence is weak. A systematic review rated the quality of evidence for continuous noise improving sleep as very low and noted that it may also negatively affect sleep and hearing [16].

It’s up to you, but don’t rely on it for medical decisions. The AASM says consumer sleep technologies aren’t validated and can’t be used to diagnose or treat sleep disorders, though they can help a conversation with a clinician [17].

CDC advises talking to your healthcare provider if you regularly have problems sleeping or notice signs of common sleep disorders [5]. The NHS suggests seeing a GP if changing your sleep habits hasn’t helped, you’ve had trouble sleeping for months, or it’s affecting your daily life [15]. Loud snoring and daytime sleepiness are worth mentioning too. For melatonin or any sleep medicine, ask your doctor or pharmacist.

No. Our content is written by the CycleRest team from the sources cited on each page, and is not reviewed by a doctor. Read our editorial policy.

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