We've all encountered children who arrive at nursery school yawning or who fail to sleep after lunch, and noticed how it impairs their motor skills, reduces their mental alertness, causes mood swings and slows their learning. And we tend to refer to it as: 'he hasn't rested enough'.
Yet while sleep is a time of rest for the body, it is certainly not a resting time for the brain! Indeed, it used to be thought that sleep was when the brain takes a long rest from activity, and merely ticks over for vital functions like breathing. In reality, our brains are extremely busy during sleep.
Between infancy and adulthood we will all spend more than one third of our lives asleep. These are far from wasted hours. It is during sleep that the body conserves and replenishes its energy stores, and that the brain re-structures itself, reprocessing the multitude of experiences stored during waking hours. Indeed, sleep has a restorative and adaptive function, and is as important as nutrition for the growth of both brain and body.
SLEEP CYCLES
The organisation of sleep states is dependent on how mature the brain is. As adults, we spend on average 2,920 hours a year sleeping! In an eight-hour night, our brain cycles about five times through one stage of Rapid Eye Movement sleep (REM) and four stages of non-REM sleep. During childhood, however, total amounts of sleep and sleep cycles are very different.
Total sleep decreases from about 16 to 17 hours a day during the neonatal period, to about 14 to 15 hours at six months, 13 hours at 24 months and 11 hours at 36 months, with a gradual shift in the distribution of day naps and night sleep. There is, of course, inter-individual variation in sleep requirements, partly due to genetics.
While there are five phases of adult sleep, infant sleep comprises only two broad states - Active Sleep and Quiet Sleep. It takes many months for children's sleep patterns to resemble those of adults.
During the first two years, sleep cycles are shorter (60 or 70 minutes versus adult cycles of 90 minutes). Moreover, whereas adults start the night in deeper non-REM sleep, infants fall directly into Active REM-like sleep, spending up to 40 minutes in this state before entering quiet sleep. This only changes at around six months of age, when infants start their sleep with Non-REM phases.
Active sleep is specifically characterised by high levels of electrical activity in the brain, together with frequent eye and body movements. Young children may seem restless, make noises, and may even open and shut their eyes when they are in the phase of active sleep. This involves lighter sleep, much easier to wake from. It is also the time when adults experience dreams, but the question of whether infants dream remains a matter of scientific debate. So, when infants are in the deep phase of sleep, loud music is unlikely to waken them, but in the active phase, the slightest whisper can cause them to stir.
The different sleep states are controlled by activity of excitatory and inhibitory neurons located in several centres of the brainstem and forebrain. Non-REM sleep is strongly dependent on integrity of neocortex and thalamus and is characterised by decreases in general metabolism and body temperature.
Although a firm link between sleep and physical growth remains to be established, scientists believe that it is during non-REM phases that growth hormones secrete. By contrast, REM phases are much like wakefulness in terms of electro-encephalic activity, a time when the brain re-organises daytime experiences into long-term memory.
During sleep, neurochemical changes inhibit the activity of the prefrontal cortex - an area responsible for directing and sustaining attention, inhibiting distraction, planning and so on. Surprisingly, however, other parts of the brain, particularly the midbrain and brain stem, are more active during sleep than during wakefulness, with different sleep cycles playing an important role in the representation and consolidation of the variety of mental images from the day's encounters.
Interestingly, the brain auto-stimulates itself during sleep, with brain activity almost doubling during REM sleep. This is true from the very start of life. Researchers have shown that the electrical activity in the newborn's brain during active sleep looks very similar to when the infant is fully conscious and stimulated! And although sleep patterns change as the child grows older, at all ages the child's senses become sufficiently desensitised to everything happening outside, thereby allowing the body and brain to concentrate on internal processes. In other words, during sleep the child's brain structures itself as much as, if not more than, during wakefulness, because of the dynamic balance between high brain activity and low physical activity, so that energy consumption can be focused on the brain.
SLEEP AND MOTOR CORTEX
Sleep forces us to conserve energy. Yet, after the very early months of life, neuronal bursts in motor cortex cause many body movements to occur during sleep, and researchers believe that these contribute to building muscle strength and mobility.
Early in life, small movements do not occur during REM sleep, but they increase substantially over the first year and beyond, suggesting a tight link between sleep movement and physical development. Once these movements occur during REM sleep, this facilitates the practice of respiration, movement, and facial expressions like smiling and frowning.
There is a common misconception that babies sleep more soundly on their tummies, prompted by the fact that they move less in this position. However, the ability to arouse relatively easily from the sleeping state is a crucial protective mechanism against respiratory complications as well as other problems such as over-heating.
By awakening spontaneously, the body is able to increase heart rate and breathing, and adjust blood pressure if necessary. That is why parents are advised to put babies on their backs to avoid Sudden Infant Death (cot death). Interestingly, scientists have found that when sleeping on their backs, babies experience more Active REM-like sleep, so crucial to brain growth.
SLEEP AND LEARNING
Children need to learn to pass from REM to non-REM sleep phases. They often open their eyes or cry out, before learning to self-soothe and not fully awaken in between phases. Often carers rush in too quickly when infants are passing from one sleep phase to another, thinking that the infants are demanding attention. Actually, they are disturbing the child's passage through different sleep cycles.
So, we can actually conceive of sleep as a cognitive process, with different sleep states playing different roles in terms of brain plasticity. In fact, it is REM sleep that seems to be particularly important for the maturation and differentiation of the central nervous system, and thus for the consolidation of memory and learning.
Research on the brains of baby birds provided vital clues to the role of sleep in brain development. Some chicks were in the early process of learning the natural song of their species by copying their mother bird's vocalisations. In the experiment, the baby birds heard and copied a taped recording of their mother's song. The researchers designed two different learning conditions for the chicks. One group of chicks practised their tweeting and were then isolated from all noise for a period of rest, during which they stayed awake. The second group of baby birds also had a silent break following their singing practice, but this time the interval was used to induce sleep.
What the scientists discovered was astonishing. The baby birds that had slept between practice sessions were able to produce a much more accurate song at the next session than those who had stayed awake during their resting time. The researchers had also measured the birds' brain activity throughout the experiment. All the brains were very active when actually mimicking their mothers' song while awake.
When it came to the silent break, however, the two groups differed. The baby birds that remained awake reduced their brain activity immediately when it was time to rest. The sleeping birds, by contrast, showed sustained levels of high activity in the brain while they slept, as if they were singing silently in their sleep and still processing their mothers' song.
The findings suggest that, while they slept, this second group of chicks was consolidating what they had just learned about their mothers' song and committing the details to memory. The chicks who stayed awake between practice sessions did not do this, so they learned the song more slowly.
Similar research was carried out on two groups of cats. Both groups initially received the same environmental stimulation. Then, one group slept six hours, while the other was kept awake and received continued stimulation for an extra six hours. They were both then tested for retention of learning. Amazingly, the group who had less training, but slept, learned far better than the group who had double the training - again attesting to the important role of sleep for the consolidation of learning.
SOCIAL DEVELOPMENT
Finally, it is important to recall the crucial role of sleep in affecting moods and affability. An overly tired child is rarely a socially interactive child. Indirectly, this then affects the behaviour of others. So a child who has sleepless nights will affect an entire family's moods and interactions, which are left at the nursery gates as the morning begins! Clearly, the role of sleep should never be underestimated throughout the whole process of development.
REFERENCES
- Annaz,D, Hill, CM, Ashworth, A, Holley, S, & Karmiloff-Smith, A (2010, in press) 'Characterisation of sleep problems in children with Williams syndrome'. Research in Developmental Disabilities
- Burke, RV, Kuhn, BR, & Peterson, JL (2004) 'Brief Report: A Storybook Ending to Children's Bedtime Problems- The use of a rewarding social story to reduce bedtime resistance and frequent night waking'. Journal of Pediatric Psychology, 29(5), 389-396
- Frank, MG, Issa, NP, Stryker, MP (2001) 'Sleep enhances plasticity in the developing visual cortex'. Neuron, 30:275-287
- Hill, CM, Hogan, AM, & Karmiloff-Smith, A (2007) 'To sleep, perchance to enrich learning?' Archives of Disease in Childhood, 92(7), 637-343
- Karmiloff, K & Karmiloff-Smith, A (2010) Getting to know your baby. London: Carroll & Brown
- Sadeh, A, Raviv, A, & Gruber, R (2000) 'Sleep patterns and sleep disruptions in school-age children'. Developmental Psychology, 36(3), 291-301.
