Sleep Calculator — Bedtime & Wake Time

Sleep runs in cycles of approximately 90 minutes, moving through light sleep, deep sleep and REM. Waking at the end of a complete cycle rather than in the middle of one tends to leave you feeling more rested. Most adults need 4 to 6 full cycles a night, which is 6 to 9 hours. Individual cycles are not identical, and the real span is closer to 75 to 120 minutes, so it is worth tracking your own pattern over 2 weeks.

The stages inside each cycle do different jobs. Light sleep, stages N1 and N2, opens and closes the cycle. Deep sleep, N3, handles physical recovery. REM is where dreaming and memory consolidation happen. Waking mid-cycle, particularly out of deep sleep, produces the grogginess known as sleep inertia, which can last 30 to 60 minutes. Waking at the end of a cycle, in light sleep, feels entirely different.

The calculator works in two directions. Give it your desired wake time and it counts back in 90-minute intervals to suggest bedtimes, or give it a bedtime and it works forward to the best wake times. Either way it allows an average sleep-onset latency of 15 minutes, the time it takes to fall asleep. If you need to be up at 7:00am, 5 cycles means 7.5 hours of sleep and a bedtime of 11:15pm, while 6 cycles means 9 hours and a bedtime of 9:45pm. Four cycles at 6 hours puts you at 12:45am, which works occasionally rather than as a routine.

The NHS recommends 7 to 9 hours a night for adults aged 18 to 64. Guidance by age runs from 14 to 17 hours for newborns and 11 to 14 for toddlers, through 9 to 11 for school-age children and 8 to 10 for teenagers, whose circadian shift makes early school starts genuinely hard, down to 7 to 8 hours for the over-65s. Around 3% of the population are true short sleepers who function on 6 hours, and most people who claim it are simply sleep-deprived. Sleep debt accumulates, so losing 1 hour a night for 5 nights leaves a 5-hour deficit that takes 2 to 3 nights of recovery to clear.

Consistency matters more than the exact total. Sleeping under 6 hours regularly is linked to obesity, cardiovascular disease and impaired cognitive function, but a steady schedule does more good than chasing a particular number. The UK average, according to ONS figures for 2024, is 6 hours 48 minutes on weekdays and 7 hours 24 minutes at weekends, roughly 30 minutes short of the NHS recommendation.

Sleep hygiene has a short list of things that genuinely work. Keep the bedtime within 30 minutes of the same point every night, weekends included, because the circadian rhythm responds to regularity more than to total hours. The NHS suggests a bedroom at 16 to 19°C, since a cool room helps you drop off. Keep it dark, as even small LEDs suppress melatonin, and quiet, ideally under 30 dB. Avoid screens for 1 hour before bed, where the mental stimulation matters as much as the blue light. Caffeine has a half-life of 5 to 6 hours, so a 7pm sleeper should stop by 2pm. Alcohol feels like it helps and then fragments the night, REM especially.

Some sleep problems need more than a routine. The NHS defines insomnia as difficulty falling asleep, staying asleep or waking too early on most nights for 3 weeks or more, and the first-line treatment is CBT for insomnia, available through a GP referral or apps such as Sleepio. Loud snoring with daytime sleepiness and observed pauses in breathing points to sleep apnoea, which affects 4% of adults and is diagnosed by a sleep study, with CPAP if confirmed. Shift work disorder comes from rotating shifts fighting the circadian clock, where strategic light exposure, melatonin and a consistent schedule on days off all help.