An uncomfortable or noisy environment
As we start to fall asleep, our muscle tone reduces and our limbs begin to relax. We may feel drowsy but our brain is still active, and any noise or discomfort can make it hard to fall asleep.
As we drift into light sleep, an area of the brain called the thalamus starts to block the flow of information from our senses to the rest of the brain. But it will still let through noises, which can wake us up.
After about half an hour of light sleep, most of us enter a type of deep sleep called slow-wave sleep. The changes in the brain neurochemistry typical of deep sleep, make it harder to be woken up. But some things will always get through - such as our names being called out loudly.
Missing out on any part of our usual cycle of sleep results in reduced quality and quantity of sleep.
An irregular routine
We all have a built-in body clock which tells us when we are tired, and helps synchronise thousands of cells in our body to the circadian rhythm.
The main synchroniser for our body clock is light. Our eyes react to the light and dark, even when our eyelids are closed.
Daylight prompts our brains to reduce the production of the sleep hormone melatonin. We become more alert, and wake up.
If we sleep less, because of going to bed late or waking up early, we're unlikely to get as much deep sleep as we need, or enough of the stage that comes after it - REM (rapid eye movement) sleep, when we do most of our dreaming.
Stimulants - coffee, alcohol, food
Caffeine is a stimulant which can stay in our system for many hours. Drinks high in caffeine make it harder to fall asleep and can result in more time in the lighter stages of sleep, with less deep sleep.
Drinking alcohol often makes us snore more, making it harder to breathe, and so making us more restless.
Although alcohol initially helps some of us fall asleep, too much of it may disrupt sleep. A lot of alcohol close to bedtime means we can go straight into deep sleep, missing out on the usual first stage of sleep.
As the alcohol starts to wear off, our bodies come out of deep sleep and back into REM sleep, which is much easier to wake from.
In the course of a night we usually have six to seven cycles of REM sleep, which leaves us feeling refreshed. However, a night of drinking means we'll typically have only one to two, and wake up feeling exhausted.
Eating a large, heavy meal too close to bedtime may also interfere with sleep. Spicy or fatty foods can cause heartburn, which leads to difficulty in falling asleep and discomfort throughout the night.
Foods containing a chemical called tyramine (examples include bacon, cheese, nuts and red wine) can keep us awake at night.
Tyramine causes the release of noradrenaline, a brain stimulant. Carbohydrates, such as bread or pasta, have the opposite effect. They trigger the release of hormone serotonin, which makes us sleepy.
The wrong body temperature
Our core body temperature goes down when we sleep. It's controlled by our body clock, which starts to open up the blood vessels of the hands, face and feet, to lose heat, as we approach the time we should be sleeping.
But if our bedrooms or duvets are too warm, our bodies can't lose heat. That can lead to restlessness and discomfort.
Our core temperature should only be half a degree less than during the day. If we get too cold, we get restless.
A busy mind
Stress is the enemy of sleep. In bed, our mind is left free to wander, and feeling anxious about getting enough sleep will only make it worse.
In these states people lose track of time. You may nod off and wake up again but it may still feel as if you are getting no sleep at all. This can result in fragmented sleep with less time spent in the deep stages of sleep.
Sleep experts recommend getting up and doing an activity which distracts our mind from worry - such as a puzzle - before trying to sleep again.
Sources: Dr Chris Idzikowski, Director of the Edinburgh Sleep Centre, Professor Jim Horne, Director of the Loughborough University Sleep Research Centre, Dr Dev Banerjee, consultant sleep physician.
