Your mum or grandma probably told you that it's bad for you to go outside in winter with wet hair because you'll catch a cold.
So now you're probably wondering if it's true that it's bad to go to bed with wet hair then? The answer to this one is a little more complex.
The good news is that sleeping with wet hair won't give you a cold or a flu. You can't actually catch a cold from being cold or damp, as the influenza virus is actually just that, a virus (meaning it is contagious).
What can happen, though, is the moisture from your hair can cause bacteria to breed in your pillow. This is bad news for those who are prone to acne, so if you're going to hit the hay with damp hair ensure you use extra pillowcases and change them every second day.
The other reason why it's considered not ideal to sleep with wet hair is that it can actually damage your strands. The hair shaft is much weaker when wet and friction from fabric such as cotton (read: your pillowcase) can rough up the cuticle, resulting in frizz, breakage and split ends.
Having said that, washing at night and letting hair dry while you sleep is a great time saver come the AM. It works easily for straight hair and can also work for wavy and curly hair types if you take some precautions.
If you shower and shampoo at night try to do so in the early evening rather than right before bed so your hair has a chance to partially air-dry. Also consider switching to a silk pillowcase -- the fibres of silk and much smoother than cotton, resulting in less friction.
To further minimise frizz or flyaways, apply a heat protectant on straight hair, or a smoothing serum on waves or curls -- both on the mid-lengths and ends before you snooze. When you wake you'll have skipped having to blow dry and can tidy any sections that need it with either a hot iron if you want a straight look or conical wand to smooth curls and waves.