Molecules like amines dominate the waking state. Adenosine builds up in our blood while we are awake and causes drowsiness and gradually breaks down while we sleep. Acetylcholine dominates the dream state.

At the beginning of sleep neurons at the base of the brain are activated to switch off the signals that keep us awake.

The brain is active during sleep. Neurons in the brainstem produce neurotransmitters such as serotonin and norepinephrine that keep some parts of the brain active. 

To begin REM sleep the pons (at the base of the brain) signals the cerebral cortex through the thalamus and signals neurons in the spinal cord to disengage the limbs. If limbs do not disengage they take part in dreams. 

Activity in parts of the brain that control emotions, decision-making, and social interaction slow down during deep sleep. Growth hormone is released during deep sleep in children and young adults. Many body cells increase production and reduce breakdown of proteins during deep sleep. This may enable repair of damage from the day’s activities.

Cytokines produced in fighting infection induce sleep so infectious diseases tend to make us feel sleepy. Sleep helps the body conserve energy and other resources needed by the immune system .

Sleeping problems are common in many disorders including mental disorders, Alzheimer’s disease, stroke, and head injury. Drugs or changes in brain function may be the cause.

Sleep deprivation may trigger seizures in people with some types of epilepsy. REM sleep seems to help prevent some seizures spreading to other brain regions, while deep sleep may promote their spread. 

People under anesthesia or in a coma have slow and weak brain waves instead of the complex, active brain wave patterns of sleep and cannot be awakened. This a different state to sleep.