Sleep Calculator

What sleep is

Sleep is a recurring state in which the body and mind are at rest: muscle activity drops, interaction with the surroundings falls away, and so does the ability to react to stimuli. That last one is a large part of what separates being asleep from being awake.

Sleep cycles

A sleep cycle is the oscillation between non-REM and REM sleep. Its timing is driven largely by hormonal signals from the circadian clock, which keeps a regular rhythm tied to outside cues such as night and day — and keeps running even when those cues suddenly change. Jet lag is that mechanism made obvious: after a long flight the clock is still set to where you left, so it is earlier or later than your body believes.

Ideally sleep follows the circadian clock. In practice it is pushed around by light, by social timing — when everyone else is awake, when work starts — by naps, and by genetics.

REM and non-REM sleep

The brain spends far less energy asleep than awake, particularly in non-REM sleep. REM — rapid eye movement — is a distinct kind of sleep, named for the eye movements, and marked by near-paralysis of the body and by dreaming. The two categories are vastly different from one another.

The body typically cycles between them over about 90 minutes, four to six times in a good night. Non-REM comes first and deepens into slow-wave sleep: body temperature and heart rate fall, the brain uses far less energy while restoring its supply of adenosine triphosphate, and growth hormone is secreted — which matters a great deal during development.

REM is usually the smaller share, and is when dreams and nightmares tend to occur. Going without it produces anxiety, irritability, hallucinations and difficulty concentrating, and the body will try to make it up: deprived of REM, later sleep shows more attempts to enter it and more time spent there once it does. That is called REM rebound, and it is consistent with REM being something the body needs. It is still not fully understood — and in some cases, depriving someone of it has transient positive effects.

Sleep quality

Quality can be measured by how hard it is to fall asleep, how hard it is to stay asleep, and how many times you wake in a night — and, more subjectively, by how rested you feel when you get up. Poor quality disrupts the cycle and the transitions between its stages.

Good quality means balancing the need for sleep against its circadian element. Ideally the timing works out so that peak melatonin and minimum core body temperature both fall after the middle of the sleep episode and before waking.

How much sleep do I need?

Sleep is far from perfectly understood, and how much a person needs varies by age and then varies again individually. The practical test is not a number: someone sleeping enough has no daytime sleepiness or dysfunction. Very generally, researchers have found 6–7 hours a night correlates with a number of positive health outcomes, though plenty of other factors bear on those outcomes too.

People tend to sleep less as they age. Newborns sleep far longer than adults, the gap narrows, and requirements start to resemble an adult's from around age 5. The CDC's recommendations:

The table is rendered from the same bands the calculator uses, so the rows it recommends and the rows printed here cannot disagree. 18–60 is a floor, not a range — the CDC says "7 or more", and the 9-hour ceiling the calculator marks against is ours rather than theirs.

How this page works the times out

Each option is a whole number of 90-minute cycles, plus fifteen minutes for falling asleep — the ten-to-twenty an average person takes, which is not zero and is the part most people forget to budget for. Waking at the end of a cycle, while sleep is light, is easier than waking out of the middle of deep sleep, which produces the disoriented fifteen-to-thirty-minute fog called sleep inertia.

Ninety minutes is a midpoint, not a constant. Cycles run roughly 70 to 120 minutes and lengthen across the night as the REM share grows. Read every time here as the middle of a window. And when the two are in tension, total sleep beats tidy phase: six cycles at the wrong moment beats four at the right one.

What this page cannot see

It knows the time you typed and nothing else — it has no idea whether you slept. That is the difference between a calculator and the app, which grades the night your ring or watch actually recorded.