Sleep in children, sleep spindles, and the metrics of memory

Authors

  • Derk-Jan Dijk Editor in Chief


If we accept that sleep is important for brain function and health and that sleep is a sensitive indicator of mental and physical health problems, then sleep in children should be of interest to many sleep researchers. The current issue of the journal contains three reports on sleep in children.

Sleep in Attention Deficit Hyperactivity Disorder

Scott and colleagues use a prospective cohort study to compare aspects of sleep of children who at one point are diagnosed with attention deficit hyperactivity disorder (n = 173) to sleep of normally developing children (>8000), i.e. normative data (Scott et al., 2013). Several interesting results from this comparison, spanning the age range of 6–140 months, are reported: total sleep time is shorter and the number of night awakenings is greater in the cases but only so after the age of 5 years. The authors point out that the effects are small and provide evidence that changes in total sleep time over time, rather than absolute levels, are better predictors of subsequent ADHD diagnosis. The study shows the power of normative data, which, however, also in this case are based on the reports of parents only.

Sleep Spindles and Development

Mikoteit and colleagues used polysomnography to investigate associations between sleep and psychological and emotional development in normally developing children aged five (Mikoteit et al., 2013). The authors also investigate how sleep behaviour at age five predicts behaviour at age six, and the children were attending kindergarten (pre-school) at both assessments. Two consecutive sleep episodes were recorded at home and sleep spindles were visually scored and only during NREM stage 2. Several associations between sleep spindles and cognitive/psychological and emotional characteristics were reported. In general, a higher spindle density was associated with positive emotional and behavioural functioning, as assessed by parents and teachers and both at age five and 1 year later. Associations appeared in general stronger in boys than in girls. These data are of interest because they underscore the predictive power of sleep, but several new questions emerge: Is a high spindle density just a consequence of adequate sleep time? Why do we only consider sleep spindles in stage 2? Are they different from the spindles in SWS? Sex differences in sleep characteristics and their correlation with behaviour are of interest and in the present sample some remarkable sex differences, and in REM sleep, in particular, are reported. But can we place these sex differences, as well as the correlations between spindles and behaviour, in a meaningful physiological or psychological framework?

Adolescent Sleep

Adolescent sleep has received much attention, and rightly so. Biological correlates of later sleep and associated sleep deprivation, be it circadian clock or sleep homeostasis related, have been discussed extensively also in this journal. King and colleagues investigate another aspect of adolescent sleep: the influence of pre-sleep behaviour on subsequent sleep (King et al., 2013). In this study, the pre-sleep behaviour was video-gaming, the duration of which (50 or 150 min) and not proximity to sleep, was the independent variable in a laboratory-based cross-over design in 17 boys 16 years of age. The extended duration video-gaming led to a 7% decrease in sleep efficiency, even though at lights-out there was no apparent difference in arousal as quantified by heart-rate. Although this study is relatively small and did not investigate the effects of video gaming per se, the study points to the need to quantify effects of evening behaviours on subsequent sleep and not only in adolescents. We know remarkably little about the effects of our evening behaviours and environment on subsequent sleep and in the absence of these data it is difficult to provide evidence based-advice.

The Metrics of Memory

A role for sleep in memory consolidation has been reported widely but the extent of sleep's role may vary between types of memory, e.g. declarative versus procedural, and be critically dependent on particular characteristics of the task. Motor skill learning has been a popular paradigm but even within this category different types of tasks may yield different results. One of those tasks relates to visuomotor adaptation, and Alboy and colleagues use this paradigm to have a closer look at sleep's role in memory (Albouy et al., 2013). The authors present seven behavioural parameters which can be used to quantify memory consolidation. They distinguish between parameters to quantify performance saving and off-line gains and the detailed presentation of these data makes very interesting reading. The effects of sleep are investigated using a delayed-retest paradigm, to avoid confounding effects of sleep deprivation induced fatigue. fMRI data complement the behavioural results. The conclusions are that ‘the influence of sleep on visuomotor adaptation skills depends on the metric used to assess performance and…’ and that ‘… data speak for the absence of sleep-dependent delayed gains in performance after motor adaptation learning, but rather for a sleep-dependent stabilization of performance’. The field of sleep and memory research has matured and these detailed analyses and future research will provide further insights into the magnitude and the nature of the effects of sleep on memory consolidation.

In Addition...

Other papers report that patients with narcolepsy-cataplexy recognize the emotions of facial expression normally(Bayard et al., 2013), despite the fact amygdala functioning may be altered in this condition, and that increased daytime sleepiness may be an early symptom in Parkinson's (Giganti et al., 2013).

All of these papers show that sleep's importance for brain function and health is increasingly recognized, but more studies in which the magnitude of these effects is quantified are required to provide evidence-based advice.

Derk-Jan Dijk

Editor in Chief