While you sleep, you sweat, and that leaves water in your mattress. Unmade beds dry faster than made beds because the water has more places to escape to the air. So if you make your bed every morning, you’ll wind up with more of these nasty things living in it because making the bed leaves the bed damper for longer, instead of letting it dry out faster unmade. Which can be a problem if you suffer from a particular kind of allergy. If you like the feeling of straightened, flat sheets when you sleep, make the bed just before you get into it.