Why you don't need to think about baby or toddler sleep architecture

This article is part of a collection inside The Possums Sleep Program called Deeper Dive, which explores the complex scientific, historical and social contexts in which families and their babies or toddlers live and sleep. You don't need to read Deeper Dive articles to be helped by The Possums Sleep Program.

Human sleep architecture is flexible and adaptive

Although there is much that scientists don't yet understand, it's clear that sleep is not a static state of consciousness, but is dynamic and changing. In babies, toddlers and adults, sleep architecture is flexible and adaptive. From an evolutionary perspective, this has been protective. Over the course of our evolutionary history, Homo sapiens was safest when sleeping socially, and when sleeping in a way that was alert to, and able to respond to, changes in the environment.

Scientists and health professionals track sleep phase changes with electrical tracings of brain activity, or with measurements of blood pressure and heart rate, or with tracings of our involuntary movements.

What happens when adults are sleeping?

When adults sleep, our brain moves between two states known as rapid eye movement sleep (REM) and non-rapid eye movement (NREM) sleep. NREM sleep is a time of relatively low brain activity. Most NREM occurs during first 3 hours after sleep onset. It has three stages.

• Stage 1 is the transition from wakefulness to sleep, characterised by intense sleepiness and brief involuntary muscle contractions.

• Stage 2 is the initiation of true sleep, characterised by decreased muscle activity and decreased awareness of outside stimulus. Adults spend around half the night in Stage 2 NREM sleep.

• Stage 3 is deep slow wave sleep, during which we are least responsive to external stimuli. This is also when night terrors, sleep walking, and bedwetting might occur.

In REM sleep, we have bursts of rapid eye movements, high brain metabolic rate, variable heart rate, suppressed peripheral muscle tone, and an absence of normal temperature regulation. Our dreams and our nightmares occur during REM sleep.

Going through the whole NREM and REM cycle takes between 90 to 110 minutes for adults, who move through NREM Stages 1-3 first, before passing into REM sleep. We don't need to get too literal about the timing of these cycles, because sleep architecture is flexible and adaptive. Over the course of the night, there are decreasing amounts of NREM and increasing amounts of REM with each cycle of sleep, and brief periods of waking between cycles, which we may not remember.

What happens when babies and toddlers are sleeping?

In babies in the first few months of life, REM sleep is different to adults, because it is characterised by restless movements. This is why we refer to REM at this age as active sleep, and NREM as quiet sleep.

When young babies go to sleep, they slip firstly in active sleep. Then they transition to deeper, quiet sleep, when your baby's limbs are relaxed and floppy with low tone, and baby's brain is in a quiet and restorative mode. In quiet sleep, babies are less likely to rouse.

Newborn sleep is described as active, indeterminate or quiet sleep, since NREM and REM sleep aren't able to be defined in the same way at this age. About half of a newborn's sleep is active at birth.

Between three to five months of age about 40% of your baby's sleep is active, and from one year of age, the proportion of active sleep is about 25-30%, closer to the proportion of REM in adults. Sleep cycles through active and quiet sleep are perhaps 45 or 60 minutes long in babies and toddlers, though still very variable, and these cycles lengthen throughout childhood. By five years of age, children's sleep architecture is much like adults.

It's normal for our babies and toddlers to sleep more some days than others

Active and quiet sleep, or REM and NREM sleep, are adaptive phases, which respond to the sleep situation that an adult or child find themselves in, including sleep debt. Sleep debt is not like owing money, to be avoided if at all possible! It's a normal part of life. In an exciting and environmentally rich situation, a small child remains happily dialled down while his sleep pressure grows very high. He'll catch up on sleep later!

This is why I've never used graphs or estimates of quiet and active sleep cycles to elucidate sleep architecture in The Possums Sleep Program. Active-quiet sleep cycles are estimates only, and thinking about them doesn’t help make sleep in your family relaxed and easy. Your own and your small child's body and brain look after their quiet and active sleep needs, depending on the day and your body's requirements. It's not something that you need to control.

You might wait until your small child is in deep sleep before you try to put her down

The only time it might be helpful to think about the difference between active and quiet sleep is when your baby or toddler is going to sleep. For the first 15 or 20 minutes after your little one is asleep (in active sleep), you’ll notice the quick little eye movements under your sweetheart’s eyelids, and funny facial expressions and occasional little jerks of her limbs.

When quiet sleep comes, your child’s little body is floppy and the rapid eye movements have stopped. It can be easier to put a baby or toddler down and have the child stay asleep when they are in quiet sleep.

This, however, is not the main game: most little ones wake when they are put down because they’ve had enough sleep to take the edge of the rising sleep pressure, and don’t need more sleep.

Your baby or toddler will take whatever sleep he needs, active or quiet, when the sleep pressure is high enough. That’s the important thing to know.