Vivid dreams that occur during rapid eye movement (REM) sleep are quickly forgotten due to a cluster of neurons that become active at this time. The recent findings from research by Nagoya University, Japan, could put an end to assumptions as to why people forget dreams.

Light sleep, or REM sleep, happens when the body rests but the brain is still active, while deep, non-REM sleep, happens when the brain is resting. A typical sleep cycle is characterised by alternating occurrences of REM and non-REM sleep; memories are also organised and processed during the cycle.

Using this understanding in a study of genetically-modified mice, the conscious activation of neurons was seen to produce a type of hormone, MCH, in the hypothalamus. The hypothalamus is the brain’s memory centre, and is a small region that regulates parameters like sleep, appetite and body temperature.

The research team noticed that activation of MCH neurons during REM sleep worsened the animals’ memory, while turning off these neurons improved their memory – the active neurons essentially curb dreams/memories, before they take root, at the hypothalamus. Furthermore, the mice performed better on memory tests when the MCH neurons were turned off during REM sleep.They thus concluded that MCH neurons eliminated memories by inhibiting hippocampal neurons during sleep.

Neurophysiology professor at the Japanese university, Akihiro Yamanaka, said their discovery could potentially lead to treatment methods for alleviating/eliminating memories that cause post-traumatic stress disorder (PTSD), a particularly debilitating condition.