Sleep plays a fundamental role in the way we learn.
Emerging evidence makes a compelling case for the importance of sleep for language learning, memory, problem solving and behaviour during childhood.
A new study that my colleagues and I have worked on illustrates how an optimal quantity of sleep leads to more effective learning in terms of knowledge acquisition and memory consolidation.
Our study has also shown that the negative impact of poor sleep on academic functioning is not always matched by a realisation of this fact by students themselves, therefore they may have little motivation to alter bad sleep habits. Unlike for adults, adolescence is a crucial time because of continual changes in the brain - so sleep is particularly important for a teenager's health.
Poor quality of sleep - caused by lots of waking up during the night - has also been reported to be a strong predictor of lower academic performance, reduced capacity for attention, poor executive function and challenging behaviour.
Many adolescents get less sleep than the average recommended level - around nine hours for this group. But due to school commitments, teenagers are required to wake up early at a set time even if they have not achieved the optimal number of hours of sleep.
Along with these early start times, teenagers also experience pubertal phase delay - meaning pubertal teenagers will sleep even less due to biological factors.
Combined with late-night activities, this can have a significant negative effect on the quality of sleep and therefore their behaviour during the day.
Insufficient and poor quality of sleep appear to be pervasive during adolescence.
These can have various consequences, such as excessive daytime sleepiness, poor diet and, in turn, impairments in cognitive control, risk-taking behaviour, diminished control of attention and behaviour, as well as poor emotional control.
MORE SLEEP OR BETTER SLEEP
In a recent study involving 48 students aged between 16 and 19, recruited through an independent sixth-form college in central London, my colleagues and I at the Lifespan Learning and Sleep Laboratory at UCL examined the link between sleep, academic performance and environmental factors.
Our results show the majority of the teenagers achieved just over seven hours of sleep, with an average bedtime at 11.37pm.
Our study shows that a longer period of sleep and earlier bedtimes - measures of sleep quantity - most strongly correlated with better results obtained by the students on a number of tests at school.
In contrast, measures that were indicative of sleep quality were mostly linked with students' performances on verbal reasoning tests and on grade point averages on tests at school. So it appears from our results that "longer sleep" is more closely related to academic performance, while "good night sleep" is more closely related to overall cognitive processing.
WHY TEENS ARE GETTING LESS AND LESS SLEEP
Our study also confirms findings from previous research showing that teenagers are getting at least two to three hours less sleep than is needed for their optimal brain development and a healthy lifestyle.
There are several modern lifestyle factors that have been shown to affect sleep. We found that consumption of energy drinks and coffee, and social media use half an hour before habitual bedtime were strongly associated with poorer sleep.
Our study has also shown that the negative impact of poor sleep on academic functioning is not always matched by a realisation of this fact by students themselves, therefore they may have little motivation to alter bad sleep habits.
Unlike for adults, adolescence is a crucial time because of continual changes in the brain - so sleep is particularly important for a teenager's health.
CONDITIONS THAT CAN AFFECT SLEEP
There is an added complexity to the sleep patterns of children with developmental disorders, despite the fact that they are more likely to suffer from sleep problems.
So far, we have examined sleep, and cognitive and behavioural functioning in children with Down syndrome, Williams syndrome and attention deficit hyperactivity disorder. All our studies show that sleep has a very important impact on cognitive and daytime functioning of children with these conditions.
When we examined levels of sleep biomarkers - melatonin and cortisol - in children with Williams syndrome, a rare genetic disorder, we found that these children had elevated levels of cortisol and dampened levels of melatonin.
High cortisol and low melatonin levels before bedtime are strongly linked to delayed sleep onset - a person taking around 50 minutes to fall asleep, against the typical 20 minutes. Since cortisol is often described as a stress hormone, high levels of this hormone before bedtime may potentially cause problems including difficulty in relaxing and falling asleep. This is an important result to consider before a child is prescribed a melatonin supplement - which might not be necessary to help solve their actual sleep problem.
The effects of the sleep disturbances extend beyond the individual. Parents of children with developmental disorders often experience heightened levels of stress and sleep problems because they are kept awake by their children.
All this shows how crucial it is for teenagers to get the right amount of sleep, otherwise it could have long-term impact on their health and on their grades.
•The writer is the director of Lifespan Learning and Sleep Laboratory, department of psychology and human development, UCL Institute of Education. This article first appeared in theconversation.com, a website that carries analysis from academics and researchers.