Why do we dream?

The most widely accepted account is that dreaming is bound up with consolidating memory and processing emotion. During REM sleep the brain is nearly as active as when awake, knitting the day’s experiences into older memories — and dreaming is thought to be what that process feels like from the inside.

The incoherence has an explanation too. In REM sleep the region that handles reasoning and planning is relatively quiet while the emotional and image-forming regions are busy. That is why impossible events feel unremarkable while you are inside the dream.

We forget most dreams because the machinery that lays down long-term memories does not run normally during sleep. That is why waking in the middle of a dream leaves you remembering it, while waking a few minutes later does not.

REM periods get longer towards morning, which is why dreams near dawn tend to be longer, more eventful and better remembered. Cutting sleep short cuts into exactly that stage — so sleeping less costs you not only rest but the dreaming itself.