Hi everyone, this week’s post follows on from last week’s, which was all about our body clock (if you’d like to read it then click here). First, I will look at the stages of sleep and the link between sleep and memory, before moving on to talk about a sleep disorder – narcolepsy.
Scientists have been able to identify the different stages of sleep by using brain imaging methods such as EEG. This measures brain potentials from the scalp to establish brain activity. Five stages of sleep have been identified – these are shown in the sleep cycle diagram shown below.
I’m sure you’ll have heard of the 5th stage – Rapid Eye Movement, or REM sleep. During this stage of sleep, all motor movement is stopped to prevent the body from acting out our dreams.
REM sleep has also been linked to memory consolidation involving our hippocampus. This is a structure in the centre of our brain which has been found to be important for long term memory. It is thought that when we are asleep, the hippocampus is involved in replaying previously activated cells, which establishes our long term memories.
Tilley & Empson (1978) tested the hypothesis that REM sleep is involved in memory consolidation by testing participants’ recall of a story after deprived stage 4 sleep, or deprived REM sleep. They found that participants who had disrupted REM sleep had significantly poorer recall than participants who had disrupted stage 4 sleep.
The involvement of sleep in helping learning has also been shown by neuroimaging studies. Maquet et al (2003) measured patterns of brain activity of participants while they practiced a reaction time motor task. They then measured their brain activity while they were asleep, and found the same patterns of activity appeared during REM sleep. Interestingly, the amount of reactivation of these patterns correlated with the extent of learning, which suggests sleep is vital for consolidating our memories and aiding learning.
This is a sleep disorder which affects 0.5-1% of the population, and is characterised by excessive sleepiness. Patients’ sleep cycles are disrupted, and they have fast entry into REM sleep (Vogel, 1976). Sudden sleepiness is usually brought on by exciting or emotionally charged events, and is often accompanied by a loss of muscle tone, called cataplexy. It is thought to be an intrusion of REM sleep into wakefulness, which is shown by one of the symptoms – sleep paralysis. Narcolepsy can be caused by a lack of a chemical called orexin, which regulates sleep (NHS Choices). This reduction in orexin has been hypothesised to be caused by cells in the immune system mistakenly attacking the cells that produce orexin, however this has not been proven.
I hope you liked this post and don’t forget to leave any requests for future posts in the comments!