Biological processes relating to sleep

• Circadian rhythms: These are natural, internal processes that regulate the sleep-wake cycle approximately every 24 hours. Our body's internal "biological clock" is situated in the hypothalamus within the brain and it responds to light cues to regulate sleep timing.

• Melatonin: This is a hormone that the brain's pineal gland produces in response to darkness. It promotes sleep by causing drowsiness and lowering body temperature.

• Adenosine: This is a sleep-promoting chemical that accumulates in the brain while awake. The increasing levels of adenosine lead to increased sleep pressure.

• Hypothalamus: This part of the brain plays a critical role in controlling sleep. It contains the 'suprachiasmatic nucleus (SCN)' which controls circadian rhythms, and the 'ventrolateral preoptic nucleus' which helps initiate sleep.

• Brain waves during sleep: During different stages of sleep, the brain produces different types of brain waves, which include alpha, beta, delta, and REM (rapid eye movement). Each of these has a different frequency and is associated with different stages of sleep.

Biological processes relating to dreams

• REM sleep: This is the stage of sleep where most dreams occur, characterised by rapid eye movements. The brain is quite active during this stage, akin to when we are awake, and it is thought that dreams are a result of this brain activity.

• Neurotransmitters: Certain neurotransmitters are active during REM sleep, such as acetylcholine, which is associated with memory and learning. A decrease in another neurotransmitter, serotonin, is also observed during REM, which may be linked to vivid dream recall.

• Activation-Synthesis hypothesis: Proposed by Hobson and McCarley, this theory suggests that dreams are a result of random brain activation during REM sleep. The brain then tries to synthesise and make meaning from these signals, forming the narrative of the dream.

• Threat simulation theory: This biological theory of dreams proposed by Antti Revonsuo suggests they are an evolutionarily adaptive mechanism which trains the brain to recognise and avoid threats in the waking world.

• Dream recall and the role of the brain: Some parts of the brain help us remember or forget dreams. The prefrontal cortex, associated with memory, is less active during REM sleep, which could explain why many dreams are forgotten upon waking.