Sleep Calculator — Bedtimes and Wake-Up Times

Pick a direction. If you know when you have to be up, the calculator subtracts six, five, four and three complete cycles from that time and then subtracts the minutes it takes you to drop off, giving four candidate bedtimes. If you know when you are going to bed, it adds the same quantities forwards to give four candidate wake-up times.

The five-cycle option is highlighted because 7½ hours sits comfortably inside the 7 to 9 hours adults are advised to get. Six cycles is 9 hours, at the top of that range. Four cycles is 6 hours and three is 4½ — both below the recommendation, and listed for the nights when the choice is between a short sleep and none.

The default 15 minutes to fall asleep is close to the adult average. Adjust it if you know your own: someone who reliably takes 30 minutes needs to be in bed half an hour earlier than the cycle arithmetic alone suggests.

About 90 minutes on average, though real cycles range from roughly 70 to 120 minutes and vary between people and across the night. Each cycle passes through light sleep, deep sleep and REM sleep:

The mix shifts as the night goes on. Early cycles are heavy on deep sleep; later ones are heavy on REM. That is why a 6-hour night is not simply three-quarters of an 8-hour night — the hours you cut are disproportionately REM.

Being woken out of deep N3 sleep produces sleep inertia: the groggy, heavy-limbed, slow-thinking state that can last from a few minutes to well over half an hour. Waking at the end of a cycle, when sleep is lightest, largely avoids it. That is the whole premise of cycle timing.

The honest caveat is that 90 minutes is an average, not a metronome. Your cycles might run 80 minutes or 105, and they lengthen through the night. Treat the suggested times as a target window of ten or fifteen minutes rather than an exact minute to set an alarm for.

Total sleep still beats clever timing. Five full cycles at an awkward hour will leave you better off than four perfectly timed ones. Use the cycle arithmetic to choose between bedtimes that are otherwise equally convenient — not to justify a shorter night.

The American Academy of Sleep Medicine and the Sleep Research Society recommend 7 or more hours a night for adults, with regularly sleeping less than 7 associated with weight gain, hypertension, diabetes, depression, and impaired immune function and performance.

Consistency matters nearly as much as duration. Going to bed and getting up at roughly the same time every day, weekends included, keeps the body clock aligned — and a stable schedule usually does more for how you feel than optimising any single night.

• Loud snoring, gasping, or witnessed pauses in breathing — the classic pointers to obstructive sleep apnoea, which is common, underdiagnosed and very treatable.
• Difficulty falling or staying asleep at least three nights a week for three months or more, with daytime consequences — the definition of chronic insomnia.
• Falling asleep involuntarily during the day, or sudden muscle weakness triggered by emotion.
• An irresistible urge to move the legs at night, usually with an uncomfortable sensation that movement relieves.
• Sleep that has changed alongside low mood, anxiety or heavy drinking — in which case the sleep is a symptom, and the screening questionnaires beside this one are the better starting point.

Cognitive behavioural therapy for insomnia, not medication, is the first-line treatment recommended for chronic insomnia in adults — and it works on the underlying pattern rather than sedating through it.

1. 1.Anchor the wake-up time first. It is the one end of the night you can control directly, and a fixed rising time drags the rest of the schedule into place.
2. 2.Move a stubborn bedtime in 15-minute steps across a week or two. Trying to fall asleep 90 minutes earlier tonight mostly produces time awake in the dark.
3. 3.Get daylight within an hour of waking. Morning light is the strongest signal your body clock receives, and it sets the timer for the following night.
4. 4.Stop caffeine six to eight hours before bed. Its half-life is around five hours, so a 4 pm coffee is still measurably present at midnight.
5. 5.Keep alcohol away from bedtime. It shortens sleep latency and then fragments the second half of the night, cutting exactly the REM the late cycles are made of.
6. 6.If you are still awake after about 20 minutes, get up and do something dull in dim light until you feel sleepy. Lying in bed frustrated teaches the brain that bed is where you lie awake.