The Sleep Cycle

There are 4-6 sleep cycles every night depending on how long you sleep. Each cycle lasts about an hour and a half and goes from NREM to REM. The REM phase makes up about 20-25% of a normal night’s sleep and NREM makes up 75-80%, depending on your age. This means you may dream for about a quarter of the time you’re asleep.

Sequences of states and stages of sleep on a typical night;

Remember stages 3 and 4 are your deep sleep. Notice how your deep sleep occurs earlier in the night. As your body prepares to wake up, your sleep becomes lighter.

Understanding your body clock

Are you more awake in the morning or the evening?

Whichever it is, it’s dictated by your biological clock, which you have no influence over. Sleep is regulated by two systems, sleep/wake homeostasis and the body clock or circadian rhythm.

Sleep/wake homeostasis is a handy little mechanism that tells us how long we’ve been awake and reminds us when it might be a good idea to take a rest.

If the sleep/wake homeostasis mechanism existed alone, we would all be most alert when we wake up, and gradually tail off through the course of the day. But it’s combined with an even cleverer natural process, the 'circadian rhythm', which spreads our periods of alertness throughout the day. Therefore the circadian rhythm can cause us to feel alert, even if we’ve been awake for hours and our sleep/wake mechanism would otherwise make us feel sleepy. If you find yourself feeling tired when you should be wide awake then it’s possible your body clock’s out of sync. Or you might just naturally be a lark who’s more awake in the morning or an owl that’s more awake at night time.

Science is also discovering that genetics play a part in whether you are a long or short sleeper, or a morning or evening person.

Circadian rhythm:

• Our internal body clock regulates sleepiness and wakefulness over a 24 hour period. This is known as the circadian rhythm.
• "Circadian rhythm" comes from Latin 'circa', meaning 'about', and 'dian', meaning 'day'.
• The circadian rhythm regulates the timing of periods of sleepiness and wakefulness throughout the day, which is why we feel more tired at certain times and not at others.
• You may not feel your best until mid-morning, while others are firing on all cylinders much earlier than that. The chances are that there’s nothing wrong with you - the circadian rhythm causes us to feel more alert at certain points of the day.

How your body knows when to sleep

Melatonin is present in high levels in the blood stream during the normal time of sleep at night and low levels are present during the waking day.

• Melatonin is produced when it goes dark and tells the body that it is time for sleep.
• Lighting can be bright enough to prevent the release of melatonin, a good reason why it’s important to dim your lighting in the evening.
• Staying awake during the hours of darkness can disrupt our natural levels of melatonin; this is why we feel jet lag or may not sleep well when working shifts.