You can plan training, meals and rest days to the gram, and still undo it by sleeping five hours. Sleep is when most of your recovery happens, and it is the one recovery habit that costs nothing. The trouble is that "get more sleep" is not a plan. A bedtime is.

How much sleep is enough?

The American Academy of Sleep Medicine and the Sleep Research Society reviewed the evidence and agreed that healthy adults should regularly sleep 7 hours or more a night. Many people feel best at 7 to 9. Needs differ from person to person, and teenagers need more than adults.

Training can raise your need a little, especially in a new programme when your body is adapting to something unfamiliar. Studies of people who cut their sleep short while lifting suggest it can blunt strength gains and make sessions feel harder, though the research is still young and effects vary. The practical takeaway is modest: treat 7 hours as the floor, not the goal, and notice how you feel at 8.

Why count in cycles?

A night is not one long block of sleep. You drift through lighter and deeper stages and then back up again, in a loop called a sleep cycle. A cycle lasts roughly 90 minutes on average, but for a given person it can run shorter or longer, and it changes from night to night. That is why "90 minutes" is a handy rule of thumb, not a law.

Waking at the end of a cycle often feels easier than being dragged out of the middle of one. You can't control this precisely, so don't chase it. Use cycles as a way to pick a sensible bedtime, then give yourself a few minutes either side.

Build your bedtime

Try pushing the wake-up time earlier for an early training session. The bedtime moves with it, and that is the real cost of a 6 a.m. workout: it is paid the night before, not in the morning. If the bedtime that comes out is not one you can live with, the honest answer is a later session, not a shorter night.

What helps you actually get there

• Keep the wake-up time steady. A regular wake time anchors your body clock more than a regular bedtime does.
• Finish hard training a few hours before bed if you can. Some people sleep fine after an evening session and some don't. Test it on yourself for a couple of weeks.
• Watch caffeine. It lingers for hours. If you are lying awake, try moving your last coffee earlier in the day.
• Dim lights and put the phone down for the last half hour. It helps the wind-down, even if it is not magic.
• Don't stay in bed awake for long. If you can't sleep after a while, get up, do something calm, and go back when drowsy.

When a plan isn't enough

Everyone has the odd bad night, and a single one will not ruin your progress. If you regularly sleep badly despite giving yourself time, snore loudly, stop breathing in your sleep, or feel exhausted all day, talk to a doctor. This calculator is a planning aid, not a diagnosis, and it can't tell you why you are tired.

On a rough night, a gentler session is often better than skipping it or forcing a hard one. Train, but leave a rep or two in reserve, and make the next night a priority.