Unit 7. Sleep

Script 15. Children's intellectual development

Bedtime stories

Regular sleeping hours really are good for children-if they are girls.

In that mythical era when children were seen and not heard, and did as they were told without argument, everyone knew that regular bedtimes were important. "Dream on!" most modern parents might reply. But research by Yvonne Kelly of University College, London, shows that the ancient wisdom is right - half the time. Daughters, it seems, do benefit from regular bedtimes. Sons do not.

Dr Kelly knew of many studies that had looked at the connection between sleep habits and cognitive ability in adults and adolescents. All showed that inconsistent sleeping schedules went hand in hand with poor academic performance. Surprisingly, however, little such research had been done on children. She and a team of colleagues therefore examined the bedtimes and cognitive abilities of 11,178 children born in Britain between September 2000 and January 2002, who are enrolled in a multidisciplinary research project called the Millennium Cohort Study.

The bedtime information they used was collected during four visits interviewers made to the homes of those participating in the study. These happened when the children were nine months, three years, five years and seven years of age. Besides asking whether the children had set bedtimes on weekdays and if they always, usually, sometimes or never made them, interviewers collected information about family routines, economic circumstances and other matters - including whether children were read to before they went to sleep and whether they had a television in their bedroom. The children in question were also asked, at the ages of three, five and seven, to take standardised reading, mathematical and spatial-awareness tests, from which their IQs could be estimated.

Dr Kelly's report, just published in the Journal of Epidemiology and Community Health, shows that by the time children had reached the age of seven, not having had a regular bedtime did seem to affect their cognition, even when other pertinent variables such as bedtime reading, bedroom televisions and parents' socioeconomic status were controlled for. But that was true only if they were female. On the IQ scale, whose mean value is 100 points, girls who had had regular bedtimes scored between eight and nine points more than those who did not.

Boys were not completely unaffected. Irregular bedtimes left their IQs about six points below those of their contemporaries at the age of three. But the distinction vanished by the time they were seven.

This difference between the sexes is baffling. Dr Kelly did not expect it and has no explanation to offer for it. As scientists are wont to say, but this time with good reason, more research is necessary.

Meanwhile, in the going-to-bed wars most households with young children suffer, the sons of the house have acquired extra ammunition. Mind you, those with the nous to read and understand Dr Kelly's results are probably not suffering from their sleep regimes anyway. (From The Economist, July 13, 2013)

Script 16. How siestas help memory

Sleepy heads

Researchers say an afternoon nap prepares the brain to learn.

Mad dogs and Englishmen, so the song has it, go out in the midday sun. And the business practices of England's lineal descendant, America, will have you in the office from nine in the morning to five in the evening, if not longer. Much of the world, though, prefers to take a siesta. And research presented to the AAAS meeting in San Diego suggests it may be right to do so. It has already been established that those who siesta are less likely to die of heart disease. Now, Matthew Walker and his colleagues at the University of California, Berkeley, have found that they probably have better memory, too. A post-prandial snooze, Dr Walker has discovered, sets the brain up for learning.

The role of sleep in consolidating memories that have already been created has been understood for some time. Dr Walker has been trying to extend this understanding by looking at sleep's role in preparing the brain for the formation of memories in the first place. He was particularly interested in a type of memory called episodic memory, which relates to specific events, places and times. This contrasts with procedural memory, of the skills required to perform some sort of mechanical task, such as driving. The theory he and his team wanted to test was that the ability to form new episodic memories deteriorates with accrued wakefulness, and that sleep thus restores the brain's capacity for efficient learning.

They asked a group of 39 people to take part in two learning sessions, one at noon and one at 6 p.m. On each occasion the participants tried to memorise and recall 100 combinations of pictures and names. After the first session they were assigned randomly to either a control group, which remained awake, or a nap group, which had 100 minutes of monitored sleep.

Those who remained awake throughout the day became worse at learning. Those who napped, by contrast, actually improved their capacity to learn, doing better in the evening than they had at noon. These findings suggest that sleep is clearing the brain's short-term memory and making way for new information.

It is already well known that fact-based memories are stored temporarily in an area called the hippocampus, a structure in the centre of the brain. But they do not stay there long. Instead, they are sent to the prefrontal cortex for longer-term storage. Electroencephalograms, which measure electrical activity in the brain, have shown that this memory-refreshing capacity is related to a specific type of sleep called Stage 2 non-REM sleep.

The ideal nap, then, follows a cycle of between 90 and 100 minutes. The first 30 minutes is a light sleep that helps improve motor performance. Then comes 30 minutes of stage 2 sleep, which refreshes the hippocampus. After this, between 60 and 90 minutes into the nap, comes rapid-eye movement, or REM, sleep, during which dreaming happens. This, research suggests, is the time when the brain makes connections between the new memories that have just been "downloaded" from the hippocampus and those that already exist - thus making new experiences relevant in a wider context.

The benefits to memory of a nap, says Dr Walker, are so great that they can equal an entire night's sleep. He warns, however, that napping must not be done too late in the day or it will interfere with night-time sleep. Moreover, not everyone awakens refreshed from a siesta.

The grogginess that results from an unrefreshing siesta is termed "sleep inertia". This happens when the brain is woken from a deep sleep with its cells still firing at a slow rhythm and its temperature and blood flow decreased. Sara Mednick, from the University of California, San Diego, suggests that non-habitual nappers suffer from this more often than those who siesta regularly. It may be that those who have a tendency to wake up groggy are choosing not to siesta in the first place. Perhaps, though, as in so many things, it is practice that makes perfect. (From The Economist, February 27, 2010)

Script 17. Restless

A strange case raises the question of what sleep is for.

The function of sleep, according to one school of thought, is to consolidate memory. Yet two Italians have no problems with their memory even though they never sleep. The woman and man, both in their 50s, are in the early stages of a neurodegenerative disease called multiple system atrophy. Their cases raise questions about the purpose of sleep.

Healthy people rotate between three states of vigilance: wakefulness, rapid eye-movement (REM) sleep and non-REM sleep. But all three are mixed together in the Italian patients. The pair were initially diagnosed by Roberto Vetrugno of the University of Bologna and his colleagues as suffering from REM behavioural disorder, in which the paralysis, or cataplexy, that normally prevents sleeping people from acting out their dreams is lost. This can cause people in REM sleep to twitch and groan, sometimes flailing about and injuring their bedmates. These patients, however, soon progressed from this state to an even odder one, according to a report in Sleep Medicine.

One of the principal ways to measure sleep is to monitor brainwave activity, which can be done by placing electrodes on the scalp in a technique known as electroencephalography (EEG). Non-REM sleep itself is divided into four stages defined purely by EEG patterns; the first two are collectively described as light sleep and the last two as deep or slow-wave sleep. When the Italian patients appeared to be asleep, their EEGs suggested that their brains were either simultaneously awake, in REM sleep and non-REM sleep, or switching rapidly between the three. Yet when subjected to a battery of neuropsychological tests, they showed no intellectual decline.

Mark Mahowald of the University of Minnesota Medical School, whose group first described REM behavioural disorder in 1986, thinks memory consolidation is still going on in the brains of the two Italian patients; hence their lack of cognitive impairment or dementia. What needs to be revised in light of their cases, he says, is the definition of sleep.

Dr Mahowald suspects that sleep can occur in the absence of the markers that currently define it, which means those markers are insufficient. What’s more, the Italian cases lend support to an idea that has been gathering steam in recent years: that wakefulness and sleep are not mutually exclusive. In other words, the human brain can be awake and asleep at the same time.

That evidence takes the form of a growing list of conditions in which wakefulness, REM and non-REM sleep appear to be mixed. An example is narcolepsy, in which emotionally laden events trigger sudden cataplexy. When the dreaming element of REM intrudes into wakefulness, which can happen with sleep deprivation, the result is wakeful dreaming or hallucinations. Since such dreams can be highly compelling, Dr Mahowald thinks they might account for some reports of alien abduction.

But there is another possible explanation of the Italian puzzle: that sleep is not necessary for memory after all. Jerry Siegel of the University of California, Los Angeles, has studied the sleep habits of many animals and thinks that could well be the explanation. All of which gives researchers something new to keep them awake at night. (From The Economist, September 20, 2008)