Today's three-minute lesson

Sleep Happens in Cycles

Sleep is not one continuous state.

Throughout the night, your brain moves through a series of sleep cycles. Each cycle contains different stages of sleep, ranging from lighter sleep to deep sleep and rapid eye movement - or REM - sleep.

Each stage plays a different role.

Deep sleep is particularly important for physical restoration, while REM sleep is associated with dreaming, memory and emotional processing. Lighter stages help your body move between them and make up a significant proportion of a normal night's sleep.

The structure of these cycles changes as the night progresses.

You tend to experience more deep sleep during the earlier part of the night. Longer periods of REM sleep usually occur towards morning. This means the final hours of sleep are not simply more of the same - they contain a greater proportion of a stage your brain still needs.

You may have heard that every sleep cycle lasts exactly 90 minutes.

Ninety minutes is a useful midpoint, but it is not a rule. A cycle can last roughly 70 to 120 minutes, and its length can change throughout the same night.

That means a sleep-cycle calculation cannot give you one perfect bedtime. It can, however, give you a loose range to test.

Begin with the time you need to wake up and work backwards. Five approximate 90-minute cycles equal seven and a half hours of sleep. Six equal nine hours. This gives many adults a useful starting range of seven and a half to nine hours before their wake-up time.

Then allow for the time it normally takes you to fall asleep. If you need to wake at 7am, for example, you might aim to be asleep somewhere between 10pm and 11.30pm. If you usually take around 20 minutes to settle, your loose bedtime range would be approximately 9.40pm to 11.10pm.

Choose a realistic bedtime within your range, keep your wake-up time consistent and notice how you feel over several mornings. Move the bedtime earlier if you are not getting enough sleep. Your energy during the day is more useful than whether the calculation looks tidy on paper.

Have you ever woken naturally feeling surprisingly alert, decided to go back to sleep and then woken again feeling groggy?

One possible reason is that your first awakening happened during lighter sleep. After drifting off again, your alarm may have caught you during a deeper stage. That heavy, disoriented feeling is known as sleep inertia.

The same thing can happen after a nap. A short nap is more likely to keep you in lighter sleep, while a longer nap gives you more chance of entering deeper sleep and waking in the middle of it. This is why a nap can occasionally leave you feeling worse rather than better.

It is also completely normal to wake briefly between cycles. Most of these awakenings are so short that you will not remember them the following morning.

Occasionally, you may become more fully aware of one.

This does not necessarily mean that something has gone wrong or that the rest of your night will be disrupted. You may simply have reached a lighter stage of sleep before beginning another cycle.

The way you respond can make a difference.

Checking the time, reaching for your phone or beginning to calculate how many hours remain can turn a brief awakening into a period of alertness. The time itself rarely helps you return to sleep, but it can give your mind something new to worry about.

If you wake during the night, try not to treat it as a problem that must be solved immediately.

Keep the room dark, avoid checking the clock and give your body an opportunity to move naturally into its next period of sleep.

Use your chosen wake-up time to calculate a loose bedtime range for yourself. Count backwards seven and a half to nine hours, then allow for the time you normally need to fall asleep. Choose one realistic bedtime within that range to test over the coming week.

Once your 30-minute bedtime alarm has sounded, put your phone aside and remove the clock from view so it is not the first thing you reach for if you wake.

If you do wake, remind yourself that brief awakenings are a normal part of moving through the night.

You do not need to manage every sleep cycle.

You only need to give them enough time and space to unfold.