The mysteries of sleep: everything we don't know about why we snooze

• Research has linked sleep with memory processing, emotional stability and even cleansing.
• Most animals have their own circadian rhythm, or body clock. Even the cells in the body have a rhythm, so it's vital for organisms to have a way to sync these.

Perhaps sleep arose to group the body's processes, ensuring that specific processes don't conflict with one another.

• One theory is that dreaming helps with sleep's memory-processing function. During sleep, the brain sorts through information gathered during the day, then decides what to keep and what connections to make between new information.
• Another theory is that dreams help with emotional processing by removing some emotional associations.
• Dreams may provide a safe way of testing the brain's reactions to negative events.
• Or dreams are just the by-product of a brain starved of external input.