I regular reader of this website (Chris) alerted me today to this BBC website report of a study which focused on the impact of sleep on memory. The study itself was conducted in mice . The animals were taught a new skill (walking along a rotating rod). Then their brains were examined overnight. During sleep, cells in the cortex of the brain (the outermost later of the brain which governs, among other things, movement) sprouted new connections. These new connections are thought to be the physical manifestation of memory. During sleep, these pathways actually fired again too, as if the brain was replaying the newly learned skill in order to consolidate it.
However, in a sleep-deprived state, these connections did not form in the same way.
The formation of new neurone-to-neurone connects during sleep has not be demonstrated before, and the researchers involved in this study believe that this observation helps explain previous evidence suggesting that sleep has an important role in the formation and retention of memory.
The researchers involved in this study went further, by attempting to ascertain what type of sleep appeared to be important for the formation of new brain cell connections. There are essentially two types of sleep:
1. REM (rapid eye movement sleep)
REM sleep is relatively shallow sleep, during which we are usually dreaming and the brain is surprisingly active. In REM sleep, brainwaves have a frequency of about 8–12 cycles per second (hertz) – about the same as when someone is awake but in a relaxed state.
2. Non-REM or ‘slow-wave’ sleep
During non-REM sleep, sleep is deeper and the brainwave frequencies are slower (often 4–7 hertz, but sometimes even slower than this). The brain is relatively quiet during non-REM sleep, but there can still be a lot going on in other departments. For example, it’s during non-REM sleep that the body secretes ‘growth hormone’ from the pituitary gland at the base of the brain.
Traditionally, adequate REM-sleep has been identified as important for memory. However, in this recent study, it was actually deeper non-REM sleep that seemed to be important for the formation of new connectivity in the brain.
However, it’s also perhaps worth bearing in mind that this study was performed in mice, and that none of its findings therefore have automatic relevance to memory function in humans.
What this study does do, I think, is underscore the fact that while sleep looks like quiet time during which not much is happening, it is in fact an active time during which processes are taking place that promote good physical and mental functioning. Part of sleep’s function is to prepare us physiologically and psychologically for the next day. Looked at like this, sleep appears less of a ‘waste of time’ and more of an essential ingredient in health and wellbeing.
1. Guang Y, et al. Sleep promotes branch-specific formation of dendritic spines after learning. Science 2014;344(6188): 1173-1178