We all sleep – some of us less than others – and yet the exact role sleeping plays on our bodies and minds has largely remained elusive.
Christoph Nissen and his team from the University Medical Center Freiburg, in Germany, compared the brain activity of 20 participants. The first study was done after a full night's sleep while the second study was carried out after a night of sleep deprivation, a total of 24 hours without any sleep.
During both experiments, the scientists applied magnetic pulses to the motor cortex, the area of the brain responsible for controlling movement, in order to activate neurons in the participants' brains.
From this, the team discovered that sleep deprivation causes significant, so-called 'connectivity changes' to occur. In particular, it was found that in sleep-deprived applicants the strength of the pulse needed to produce a muscle response in the left hand was much lower for the sleep deprived participants, suggesting brain excitability was higher after lack of sleep.
Brain excitability refers to the strength of reactions of the brain to a given stimulus or irritation. It is believed that brain excitability reflects the overall strength of connectivity in the area of the brain it is targeting, therefore when it is affected by something like sleep deprivation, excitability changes meaning in a way that causes the strength of synapses in the brain to change.
This explains why people feel less alert and unable to complete simple tasks when they're sleep deprived. Although this sounds obvious, it is the first time a direct link between the two has been visualised in this way.
Brain excitability is supposed to be higher after sleep deprivation because a lack of sleep usually leads to increased synaptic strength in the brain. There is an overall increase in synaptic strength across wakefulness, therefore a major function of sleep is to lead to a synaptic downscaling. Without this downscaling, synaptic strength keeps increasing, affecting how the brain reacts and responds to stimulus when it's awake and throwing reactions out of sync.
The control of movement after sleep deprivation was measured because this is the best-validated practice and most accessible way to measure brain reactions in humans.
In addition, sleep-deprived individuals performed poorly on a word-pair memory task. Participants were required to learn a total of 46 pairs, before having the word pairs presented to them in the first round. In the second round, the participants had to remember the second word on presentation of the first.
Participants were measured on how many words they correctly remembered, of which for the sleep-deprived individuals, there were fewer correct answers than when they had had a full eight hours' sleep the night before.
The study suggests that sleep is needed in humans in order to recalibrate brain function because the brain needs to take a rest from synaptic strength in order to improve memory skills and recalibrate our synapses.
Earlier this month, scientists pinpointed that a 'switch' in our brains tells us when to go to sleep and when to wake up, known as the sleep homeostat.
In addition, the effects of sleep deprivation have been proven to be particularly detrimental to astronauts’ performances in training and during space flight.
Whilst getting at least eight hours sleep a night is recommended to avoid sleep deprivation, too much sleep can lead to sleep drunkenness whereby having a lie-in can confuse the part of your brain that controls your body's daily cycle.