Falling asleep happens as the result of a combination of effects inside our bodies and triggers from outside.
Circadian rhythm – also known as the body clock, this is the rhythm that helps us to follow a routine of waking and sleeping over 24 hours. External factors in the world around us (known as zeitgeibers) fix us to this rhythm – these include temperature, mealtimes, social activities and light.
In the mornings, daylight triggers the production of a hormone called cortisol, which wakes us up and makes us feel alert. At the end of the day, when it starts to get dark, our cortisol levels drop and another hormone, melatonin (sometimes known as the ‘sleepy hormone’), increases helping us to feel ready for sleep.
Sleep pressure – sleep pressure builds up gradually from the time we wake up, eventually leading to us feeling tired and ready to sleep. In a good sleep routine, this happens at night-time, at the same time as melatonin hits its peak.
What does a good sleep look like?
We usually go into sleep through NREM (Non-Rapid Eye Movement) sleep, and work through 3 steps or stages
NREM Stage 1 – light sleep
NREM Stage 2 – slightly deeper sleep
NREM Stage 3 – the deepest stage of NREM sleep
NREM sleep is shown in blue on the hypnogram. Even though it’s when we can be in our deepest sleep, our bodies will still be working hard releasing hormones and renewing and repairing tissues.
After NREM sleep, we usually go into a phase of REM (Rapid Eye Movement) sleep, shown in red on the hypnogram. REM sleep is a lighter, drowsy sleep and it’s when memory consolidation happens and often when we dream.
In a good night’s sleep, we cycle through each of these stages 4 or 5 times, with each cycle lasting around 90 minutes in an adult.
Each stage of sleep provides us with one or more of the benefits described above; if we lose out on sleep at any stage, we miss out on its benefits. As the hypnogram shows, if we sleep for fewer hours, we miss out on the benefits of chunks of REM sleep and NREM Stages 1 and 2 sleep.
Does sleep change over our lives?
- Babies in the first three months of life often go straight into REM sleep and have frequent naps. This helps them with memory consolidation.
- Teenagers tend to feel sleepy later at night, and want to sleep in later in the mornings, because of hormonal changes. Find out more about teenage sleep in our Teen Zone.
- Children of different ages need different lengths of sleep at night. You can find guidelines for your child’s age-group here.