Sleep Calculator

Waking up in the middle of deep sleep is what leaves you groggy, a state researchers call sleep inertia. Sleep moves through cycles of roughly 90 minutes, and you tend to feel more refreshed waking near the end of a cycle, when sleep is lighter. Enter the time you need to get up, and the calculator counts back whole cycles, plus the time it takes you to drift off, to suggest bedtimes. To wake at 6:00 AM after six cycles, aim to be in bed at 8:45 PM; for five cycles, 10:15 PM. Switch to go to bed at to see good wake-up times from a given bedtime instead.

Cycle length varies from person to person and through the night, typically between about 70 and 120 minutes, so treat the times as a starting point and adjust the cycle length if you consistently wake before your alarm.

Sleep cycles

A night's sleep is not one uniform state. The brain moves through a repeating sequence of stages: light non-REM sleep, deeper non-REM sleep, then a period of REM (rapid eye movement) sleep, after which the cycle starts again. Most adults complete four to six cycles a night. The balance shifts as the night goes on: early cycles contain the most deep sleep, while REM periods become longer toward morning, which is why dreams are often vivid just before waking.

REM and non-REM sleep

Non-REM sleep has three stages. Stage N1 is the brief transition from wakefulness, lasting several minutes, in which breathing and heart rate slow and muscles relax; people woken from it often say they were not asleep at all. Stage N2 is light sleep and makes up about half of a typical night. Body temperature drops, eye movements stop, and the brain produces bursts of activity called sleep spindles that are linked to memory consolidation. Stage N3 is deep, slow-wave sleep, when brain waves are slow and large and it is hardest to wake someone. It is important for feeling rested, for physical recovery and for immune function, and it decreases with age.

REM sleep usually begins about 90 minutes after falling asleep. The eyes move rapidly behind closed lids, brain activity approaches waking levels, and most vivid dreaming happens. Breathing and heart rate become faster and less regular, while the major muscles are temporarily paralyzed, which keeps people from acting out their dreams. REM sleep is thought to play a key role in emotional processing, learning and memory. Babies spend about half their sleep in REM, compared with roughly a fifth to a quarter for adults.

Sleep quality

How well you sleep matters as much as how long. Signs of good sleep quality include falling asleep within about 30 minutes, waking no more than once a night, falling back asleep quickly, and spending most of the time in bed actually asleep. Regular bed and wake times, a dark, quiet and cool bedroom, daylight and exercise during the day, and avoiding caffeine late in the day and alcohol or large meals before bed all help. Loud snoring with pauses in breathing, or persistent trouble sleeping, are reasons to see a doctor.

How much sleep do I need?

Sleep needs change across the lifespan. The ranges below are the recommendations published by the CDC, based on guidance from the American Academy of Sleep Medicine and the National Sleep Foundation. Adults who regularly get less than 7 hours have higher risks of obesity, diabetes, high blood pressure, heart disease and depression, and short sleep slows reaction time as much as some levels of alcohol.

Individual needs vary within each range, and older adults often sleep more lightly and wake earlier but do not need dramatically less sleep. If you need an alarm every day and sleep much longer on days off, you are probably not getting enough during the week.

Frequently asked questions

Sources

• CDC – About Sleep
• NIH NINDS – Brain Basics: Understanding Sleep
• NHLBI – Sleep Deprivation and Deficiency