Sleep Calculator

Use the calculator here to work out when to go to bed or wake up so that you wake up feeling rested, after completing a sleep cycle.

Sleep Cycles

A sleep cycle can be described as the back-and-forth switching between non-REM (rapid eye movement) sleep and REM sleep, both of which are explained below.

The timing of sleep is mostly controlled by hormonal signals coming from the circadian clock. The circadian clock follows a regular rhythm that matches outside cues (such as night and day), and this rhythm can continue even if those outside cues suddenly vanish. Jet lag is a good example of this: when a person travels quickly across long distances, their body's circadian rhythm is thrown off, leaving them out of sync with the local time. As a result, they feel as though it is either later or earlier than what their body expects, which affects their sleep.

Ideally, a person's sleep cycle should follow the circadian clock, but sleep can be influenced by many things, including light, social timing (when others are awake, when work is required, etc.), naps, genetics, and more.

While a person sleeps, the brain uses much less energy than when they are awake, especially during non-REM (rapid eye movement) sleep. REM sleep is a type of sleep defined by several features, including the eye movements it is named after, a kind of temporary paralysis of the body, and the presence of dreams. Non-REM and REM sleep are two categories of sleep that are very different from each other.

Normally, the body alternates between non-REM and REM sleep over a period of about 90 minutes on average, and this should happen 4–6 times during a good night's sleep. Non-REM sleep comes first, eventually progressing into slow-wave sleep, or deep sleep. During this stage, body temperature and heart rate drop, and the brain uses much less energy while rebuilding its supply of adenosine triphosphate (ATP), a molecule used to store and transport energy. Growth hormone is also released during slow-wave sleep, which is especially important for human development.

REM sleep usually makes up a smaller share of total sleep time, and it is best known as the stage when dreams or nightmares tend to happen. Being deprived of REM sleep can lead to anxiety, irritability, hallucinations, and trouble concentrating. Studies have shown that when the body is deprived of REM sleep, later sleep will involve the body making more attempts to enter REM sleep, as well as spending more time in REM sleep compared to periods without sleep deprivation. This is known as a REM rebound, and it supports the idea that REM sleep is necessary for the body. REM sleep, its effects, and its necessity are still not fully understood, and although it is generally considered an important and necessary part of sleep, in some cases, REM sleep deprivation can have temporary positive effects.

Sleep quality can be measured by how difficult it is for a person to fall asleep and stay asleep, as well as how many times they typically wake up during a single night. It can also be measured more subjectively by how rested a person feels when they wake up.

Poor sleep quality disrupts the sleep cycle and the transitions between the different stages of sleep. To have good sleep quality, the need to sleep must be balanced against the circadian aspect of sleep. Ideally, the timing of sleep should be balanced so that the highest concentration of the hormone melatonin and the lowest core body temperature occur after the middle of the sleep episode and before waking up.

Sleep is far from fully understood, and how much sleep a person needs can vary greatly depending on certain factors, such as age, as well as being entirely individual. A person who gets enough sleep should not experience daytime sleepiness or dysfunction.

Very broadly speaking, researchers have found that getting 6–7 hours of sleep per night is linked to a number of positive health outcomes, but there are also many other factors that may affect these outcomes.

As people get older, they tend to sleep less, with newborns sleeping for significantly longer hours than adults. This difference shrinks with age, and sleep requirements become more similar to those of adults starting around age 5.

Below are the CDC's recommendations for how many hours a person should sleep based on age.

Age Group Recommended Hours of Sleep Per Day

0–3 months 14–17 hours

4–12 months 12–16 hours per 24 hours (including naps)

1–2 years 11–14 hours per 24 hours (including naps)

3–5 years 10–13 hours per 24 hours (including naps)

6–12 years 9–12 hours per 24 hours

13–18 years 8–10 hours per 24 hours

18–60 years 7 or more hours per night

61–64 years 7–9 hours

65 years and older 7–8 hours