Wake-time planner

Most sleep advice starts at bedtime, the end of the night that is hardest to control. This planner starts at the other end. Enter the time the alarm goes off on a workday and the time you surface on a free day, and it works out the rest.

How to read the result

Asleep by is your workday alarm minus the sleep you chose. In bed by adds a quarter of an hour for falling asleep, which is a planning allowance, not a measurement of you.

Free-day alarm is the workday alarm plus one hour. The gap between the two wake-up times is the part worth watching. Researchers call it social jetlag: the body keeping one schedule for work and another for days off.

Daylight is the half hour after the workday alarm. Morning light is the cue that pulls the body clock earlier, which is what makes the earlier bedtime feel possible a few days in.

What it is based on

The sleep amounts on offer start at seven hours because the American Academy of Sleep Medicine and the Sleep Research Society recommend seven or more hours a night for adults on a regular basis. Where you land above that is individual; pick the amount after which you wake without an alarm on a long holiday.

Social jetlag was described in a 2006 study of 501 people, which found the largest workday to free-day differences among people whose natural schedule runs late. The planner uses wake-up times as a simple stand-in; the researchers measure it from the midpoint of sleep, so treat the number as an approximation.

The daylight line follows a review of how light affects the body clock: light in the morning advances the clock, light in the evening and at night delays it.

What it cannot tell you

The one-hour cap is a rule of thumb, not a research finding. No study hands out a magic number; an hour is small enough to keep Sunday night intact and large enough to still feel like a lie-in. The planner also knows nothing about shift work, small children or health conditions. If you sleep badly no matter what the schedule says, that is a conversation for a doctor, not a calculator.