What time should you go to bed? Work back from your alarm

The best time to go to bed is your wake time minus the amount of sleep you need, minus the time it takes you to fall asleep. Most adults need at least seven hours, so if your alarm goes at 7 AM and you need about eight hours, aim to be in bed by about 10:30 PM.

There is no single right bedtime for everyone, because wake times, sleep needs and body clocks all differ. The sums are the easy part. Keeping the bedtime once you have it is harder.

How to calculate what time to go to bed

1. Start with your wake time. It is the fixed point, set by work, the school run or a dog that wants a walk. Use your weekday time, and plan to keep it on weekends too, give or take an hour.
2. Decide how much sleep you need. The American Academy of Sleep Medicine recommends at least seven hours a night for adults, and the National Sleep Foundation suggests seven to nine for most adults. If you don't know your number, start at eight and adjust (more on that below).
3. Add time to fall asleep. Few people drop off the moment their head hits the pillow. Allow about half an hour between getting into bed and the sleep you are counting, which covers settling in and a page or two of a book.
4. Count back. Wake time minus sleep minus that half hour is when you should be in bed. Start winding down about an hour before that.

What time to go to bed for your wake time

Here are the in-bed times the sums give, with half an hour allowed for falling asleep.

If the right-hand columns look impossibly early, that is common, and it usually means your evenings are doing the deciding. Pick the column that matches what you need, not the one that matches your current habits.

How much sleep do you actually need?

For most adults, somewhere from seven to nine hours. Teenagers need more, and needs vary from person to person, which is why the bodies above give ranges rather than one number.

The most reliable way to find your own is to see what happens without an alarm on a week off, once the first few long nights are behind you. How many hours of sleep do you need? explains that test and a simpler sleep diary. Day to day, these signs are clear enough:

• You probably need more if you can't get up without the snooze button, fade badly in the afternoon, or sleep far longer on days off than on workdays.
• You are probably close if you wake around the alarm, stay reasonably alert until evening, and feel much the same on Monday as on Friday.

If you regularly get eight hours or more and still feel exhausted, raise it with a doctor rather than trying to solve it with an earlier bedtime. Can you catch up on sleep on weekends covers what to do if you are already short.

What about sleep cycles and 90 minutes?

Many bedtime calculators count back in 90 minute blocks, so that the alarm lands at the end of a sleep cycle. The idea has something to it: sleep does move through cycles of light, deep and dream sleep, each roughly an hour and a half long, and waking from deep sleep feels worse than waking from light sleep.

The catch is that cycles vary in length from person to person and through the night, and so does the time it takes you to fall asleep. A calculator can't know where you will be in a cycle at 6:45. Use one as a rough guide if you like, but getting enough sleep and keeping the same times every day matters more than hitting a cycle boundary.

What if you're a night owl?

Body clocks differ. Some people are sleepy early and up with the sun, while others come alive in the evening and struggle to function before nine in the morning. Which way you lean is partly inherited, and it shifts with age: teenagers and young adults tend to run late, and many people drift earlier as they get older.

There is no good evidence that one clock time is right for everyone, or that an hour before midnight is worth two after it, as the old saying goes. What matters most is enough sleep, at a time that suits your body clock as far as your life allows, at roughly the same time every day.

If you are a night owl with an early job, you can move your clock earlier, slowly:

• Shift in small steps, about 15 minutes earlier every few days. Jumping an hour at once usually means an hour lying awake.
• Get bright light early. Go outside soon after you wake. Morning light is the strongest signal your clock gets.
• Keep evenings dim. Lower the lights and the screen brightness in the last hours before bed.
• Hold the line at weekends, within about an hour of your weekday times. Two late nights can undo a week of progress.

Setting a bedtime you'll actually keep

A bedtime on paper is easy. The hard part is 10:15 on a Wednesday, when you are deep in something and the time hasn't crossed your mind.

• Set a wind-down cue, not just a bedtime cue. An alert at bedtime itself comes too late. The useful one comes an hour before. How to set a bedtime reminder on your Mac walks through Reminders, Screen Time Downtime, a Sleep Focus and Night Shift.
• Keep the same time every day. Regularity is a large part of the benefit, and it makes falling asleep easier.
• Remove the easy excuses. Charge your phone outside the bedroom, stop work at a set time, and keep the last hour off email.

If you use Notchpals, the wind-down cue can come from the top of your Mac's screen. Bedtime is one of the things it asks for during setup (the default is 11 PM, and you can change it later in Settings). With Sleep in your crew, a character comes out of the notch once in the hour before that time, but only if you are still at the Mac. Boo, a ghost who would rather not make a fuss, says it like this:

BooBedtime soon... then it's my turn to look after the house. You can rest.

Keep working past it and Boo comes back once more, and there is one last visit if you are still up in the small hours. If you have already closed the lid, none of them come. Give it the bedtime from the table above, not the one you happen to keep now.