The huge complexity of what happens when we are asleep explains why disentangling the neuroscience of sleep has been so difficult, and why sleep patterns can be so variable. Sleep habits are the product of environmental stimuli and endogenous interactions. ‘The sleeping brain’ reviews the major brain structures and neurotransmitter systems involved in the generation of sleep in mammals. In the 1930s, studies on monozygotic and dizygotic twins seemed to show that genes have a strong influence on sleep patterns, but to what extent? Currently, it seems unlikely that there is a unique sleep neurotransmitter or brain structure. During sleep we activate this network in a different way.