About sleep

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Why sleep?

Humans will spend about one-third of a lifetime asleep. Yet science knows remarkably little about why we need to sleep, what happens when we sleep, or why we feel restored by a good sleep.

We do know that sleep deprivation has serious consequences. This is because a lot of activity takes place in the body and the brain during sleep. For example, when it’s dark, our bodies produce extra melatonin to grow and repair cells and muscle tissues. Our bodies start to feel sleepy in response to the melatonin.

Normal sleep phases

Normal sleep in all humans involves phases of:

• active and quiet sleep
• light and deep sleep.

Active and quiet sleep
In active sleep, babies breathe irregularly and shallowly, twitch their arms and legs, and display rapid eye movements under closed lids. In fact, limb muscles become temporarily paralysed. Heart rate increases, and blood pressure rises.

In grown-ups, active sleep is called REM (rapid eye movement) sleep. Active sleep is a form of light sleep.

In quiet sleep, babies lie still and breathe deeply. They will occasionally jerk or ‘startle’. Quiet sleep is similar to non-REM sleep in grown-ups. It can be light or deep.

Light and deep sleep
Light sleep can be very light – sometimes you might not even think you are sleeping. We can be easily woken from light sleep.

Someone in deep sleep is hard to wake. Deep sleep is a lot more peaceful and restful. It is thought that deep sleep is the time when we grow and heal.

Sleep cycles

In the course of one night, grown-ups and children move through repeating cycles of quiet and active sleep.

Typically, a larger proportion of our sleep in the early part of the night is quiet sleep (about 80%). Then, about halfway through our normal-length sleep, our sleep cycle flips. By morning, about 80% of our sleep is active sleep.

This is why it’s easier to be disturbed towards the end of a sleep.

Sleep cycles in babies and children
Cycles of quiet and active sleep last 30-50 minutes in babies.

Some babies and children fall deeply asleep very quickly. Others sleep lightly, fidgeting and muttering for up to 20 minutes, before getting into deep sleep.

Children usually wake briefly at the end of each sleep cycle. This is a normal part of healthy sleep – all children do it. Some children call out when they wake and need help settling again. Independent sleepers can put themselves back to sleep. Not all parents hear their children when they wake.

Grown-up sleep cycles
In grown-ups, each cycle of active and quiet sleep lasts about 90 minutes. As in children, each cycle ends in a brief awakening.

Brief wakings between sleep cycles normally do not disrupt our sleep. If things have not changed, we normally go straight back to sleep and usually don’t remember having woken in the morning. If things have changed around us, we are uncomfortable, our pillow is missing, or we are disturbed by a noise, we might wake fully.

How sleep cycles change over time

The amount of time we spend in each phase of sleep varies depending on our age.

At birth, full-term infants spend about half their sleeping time in active sleep. Each sleep cycle lasts only 40 minutes (compared to 90 minutes in a grown-up). This means that, biologically, infants are programmed to sleep more lightly and have more brief awakenings than grown-ups.

The amount of active sleep in our sleep cycle decreases with age. By three years of age, 33% of sleep is active. By the time we are teenagers, only about 20% of our sleeping time is active sleep.

When we sleep

It takes time to consolidate most of our sleep into the night-time.

Under six months

• Newborns sleep on and off through day and night.
• Babies aged 3-6 months might start moving towards a pattern of 2-3 daytime sleeps of up to two hours each. They might still wake at least once at night.

From 6-12 months

• From about six months, babies have their longest sleep at night.
• Between six and 12 months, most babies are in bed between 6 pm and 10 pm. They usually take less than 30 minutes to get to sleep (but about 10% take longer).
• Most babies can sleep for a period of six hours or more at night and are waking less. About 60% will wake only once during the night and need a grown-up to settle them back to sleep. About one in 10 infants will call out 3-4 times a night. More than a third of parents report problems with a baby’s sleep at this age.
• Around 85-90% of infants aged 6-12 months are still having daytime naps. Naps usually last 1-2 hours. Some infants will sleep longer, but up to a quarter nap for less than an hour.

From 12 months
From this age, children tend to sleep better. Some toddlers start to resist going to sleep at night, preferring to stay up with the family. This is the most common sleep problem reported by parents. It peaks around 18 months and improves with age.

Less than 5% of two-year-olds wake three or more times overnight.

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