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.

What we do know is that sleep deprivation has serious consequences and that sleep is necessary for the brain and body to develop. Undoubtedly, this is one of the reasons why very young children spend so much time sleeping. 

Although sleep is thought of as a time to rest and recuperate, 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.

What is normal sleep?

• All humans cycle through stages of light and deep sleep.
• During quiet sleep (similar to adult non-REM sleep), a baby will breathe deeply and evenly and lie still with an occasional body jerk or ‘startle’.
• During active sleep (similar to adult REM or rapid eye movement sleep), a baby will breathe irregularly, twitch her arms and legs and display rapid eye movements under closed lids.
• Cycles of quiet and active sleep last 30-50 minutes in babies and up to 90 minutes in adults.
• Typically, a brief awaking occurs at the completion of each cycle. An independent sleeper puts herself back to sleep after one of these brief wakings.

More about quiet and active sleep

Sleep cycles

In the course of one night, adults and children move through repeating cycles of quiet and active sleep. For an adult, each cycle lasts about 90 minutes and 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, then we might wake fully. Waking during the night is a normal part of healthy sleep – all children do it. It’s just that not all children call out and not all parents wake when their children do.

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

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 an adult). This means that biologically, infants are programmed to sleep more lightly and have more brief awakenings than an adult. 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.

Newborns sleep on and off through day and night. 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 about six months, babies have their longest sleep at night. Between six and 12 months, most babies are in bed between six and 10 pm and 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 an adult to settle them back to sleep. About one in 10 infants will call out 3-4 times a night. Waking up is partly related to the child’s worry about being separated from a parent or parents. Overcoming this worry is normal and necessary for all children as a step towards becoming more independent at night. More than a third of parents report problems with a baby’s sleep at this age.

From the age of 12 months, 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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