Because our brains lose their capacity for balance and control. In a healthy brain after plenty of sleep, the evolutionarily older parts of the brain are controlled by various parts of the neocortex (the outer convoluted part of the brain that is especially large in humans).

These older parts are similar in many other animals and include circuits specialised for emotional behaviour, including the amygdala. This responds when people look at emotional stimuli, and brain scans show a 60 per cent increase in reactivity when people are sleep deprived. Adolescents frequently get insufficient sleep and this contributes to emotional and behavioural difficulties.

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