Why do we dream?

Every night, for roughly two hours in total, your brain creates stories you did not ask for. Dreaming mostly happens in a sleep stage called REM — rapid eye movement — when your eyes dart about under closed lids and your body is kept still so you do not act out the dream.

Why? One strong idea is memory. During REM sleep the brain replays the day, keeps what matters, and files it away. Dreams may be what that sorting feels like from the inside.

A second idea is emotional rehearsal. Dreams let us practise fear, conflict and loss in a safe place, which may be why so many dreams are about being chased, falling or being late for an exam.

Whatever the reason, lose REM sleep and your memory and mood suffer the next day. Dreams, it seems, are work — just not the kind you remember doing.