Mind Hacks (here) has reported on a recent study of how the brain goes to sleep. M. Magnin and his group asked permission of patients with inserted electrodes (in preparation for epilepsy surgery) to record from the electrodes during normal activities. In this case the activity was falling asleep.
They found that as people drifted off to sleep, the deep brain area the thalamus wound down several minutes before the cortex. This is surprising because the thalamus has traditionally been considered a structure that regulates alertness and ‘relays’ information to the rest of the brain from the body and the spinal cord. It was often assumed that it would ‘shut down’ the cortex first, because this is often considered to be where our ‘higher’ conscious functions like abstract thought and complex perception lie, while continuing with its minimal vigilance functions. A bit like a neural ‘standby’ setting. Instead, what seems to happen is that the thalamus ‘disconnects’ itself and leaves the cortex freewheeling before it finally settles down into inactivity. Indeed, freewheeling is, perhaps, a good description here. The researchers found lots of uneven activity in the upper brain areas as they were left to drift off. Interestingly, sleep onset is one of the times when we are most likely to experience hallucinations. In fact, they are so common as to have been given their own name - hypnagogic hallucinations - while this drifting off period is known as hypnagogia.