Some of what happens overnight sounds almost unbelievable, and here are 15 of the strangest and most fascinating. During deep sleep, it flushes much of that away through a remarkable cleaning system scientists only fully recognised in the past decade. That’s because your brain spends the night deciding what to keep, what to strengthen and what can safely be forgotten. Your immune system gets ready for tomorrow’s germs While you sleep, your body produces infection-fighting cells and proteins that help protect you against viruses and bacteria. During REM sleep, your brain processes emotional experiences, helping you make sense of the day and sometimes putting things into perspective.