Most of us know that a bad night’s sleep changes our mood the next day. We even recognise it in other people. But why does a lack of sleep make us so emotionally reactive? When your brain is sleep-deprived it swings you excessively between the emotional extremes of positive and negative moods. Sleep loss contributes to different types of mental illness, including depression, anxiety and schizophrenia. And mental illness can interfere with your inability to sleep well. You can combat the effects of sleep loss and insomnia with cognitive behavioural therapy (CBT) - the most promising treatment available for sleep deprivation.