So how does this combined action of circadian rhythmicity and sleep homeostasis work? Well, during the biological day the circadian clock generates an alerting or wakefulness promoting signal that becomes stronger as the day progresses and reaches maximum strength in the evening. This may seem a bit paradoxical, but this signal needs to become stronger as the day progresses because sleep pressure also increases the longer we’re awake – so something needs to keep us alert.
But as we enter the biological night, the wakefulness promoting circadian signal dissipates and turns into a sleep promoting signal with a maximum strength at around 6-8am. Again, this may seem a bit paradoxical but under normal conditions when we sleep at night, this comes in handy because the sleep promoting signal allows us to continue to sleep well even after six or seven hours when the sleep pressure has dissipated.
Problems arise when we stay awake at night and the next day, however. During the night, sleep pressure remains high and even increases because we are awake. The circadian signal no longer opposes this pressure and we struggle to stay awake and to perform. The next day, the circadian clock, which still ticks whether we are asleep or not, starts promoting awake signals again so it becomes a little bit easier to perform and stay awake.
What does this look like in the brain?
This is all fine and good and makes sense. Indeed, this working model is widely accepted from what we’ve seen happen when it comes to behaviour. But what does this combined action of circadian rhythm and sleep homeostasis look like within the human brain?
Our team of researchers, from the University of Liege and the University of Surrey, scanned the brains of 33 people using functional magnetic resonance imaging (fMRI) – which gives a detailed picture of levels of neuronal activity throughout the brain – who were sleep deprived over two days and following a period of recovery sleep. We also measured melatonin levels to have a good indicator of internal biological time, which varies between individuals. Our results are published in Science.