How two body clocks manipulate your sleepy mind
Different brain regions tick along to circadian rhythm and sleep pressure in different ways. Amy Middleton reports.
After a sleepless night of tossing and turning, do you really struggle to concentrate in the early morning but seem to be able to focus better as the day wears on, particularly in the evening?
A new study published in Science explains this gradual improvement: researchers in the UK and Belgium found different regions in our brain are affected by sleeplessness and the pressure to sleep in different ways.
“Our data may ultimately help us to better understand how the brain maintains performance during the day, why many symptoms in psychiatric and neurodegenerative conditions wax and wane, and why in the early morning after a night without sleep we struggle to maintain attention, whereas in the evening it is not an issue,” says co-author Derk-Jan Dijk from the University of Surrey in the UK.
It’s long been understood that a lack of sleep affects brain function – but how our cognition deteriorates doesn’t simply follow the amount of time since our last nap. During our biological awareness of “daytime”, for instance, it seems we can function better with no sleep, than during the biological night.
So to untangle how our innate awareness of time – our circadian rhythms – and our drive to sleep – our homeostatic sleep drive – play their own separate parts in this interaction, Vincent Muto from the University of Liege in Belgium and colleagues kept 33 participants awake for 42 hours.
They gave their participants 12 cognitive tests of varying difficulty throughout the time and scanned their brain with fMRI to see which parts lit up during the tasks.
The scan and activity combo was repeated one more time after a good night’s recovery sleep, to measure how the brain bounces back.
The scans were clustered around the times of day at which circadian rhythms are known to affect our cognitive ability – first thing in the morning and the first few hours of the night up until 1am. Levels of melatonin – the hormone released by the brain that helps us fall asleep – were also monitored.
Muto and his crew found cognitive functionality was, indeed, affected in different ways across the brain. In the cortical regions or grey matter, for instance, responses were most shaped by circadian rhythms – that is, our biological awareness of an ongoing, almost 24-hour clock – as well as the amount of time since the last sleep.
Some regions beneath the cortex were influenced by circadian rhythms but not significantly by the amount of time since sleep. And other regions, including the frontal areas, were most disturbed by sleep deprivation.
Interestingly, the level of cognitive deficit also depended on the type of task being performed. For more complex memory tests, people were on the whole less affected by sleeplessness compared to more simple, reaction-time tasks.
“It is very gratifying to see directly at the level of fMRI-detected brain responses that circadian rhythmicity and lack of sleep both have such a profound influence on brain function,” adds Dijk.
The researchers argue that sleeplessness is an increasing part of life in our society, and it’s important to understand how it moulds human ability, and particularly the likelihood of error or misjudgement.
“Our findings have implications for the understanding of the brain mechanisms underlying the maintenance of daytime cognitive performance and its deterioration,” the researchers write, “as observed in shift work, jet lag, sleep disorders, ageing and neurodegenerative diseases.”