Drying Hair Before Sleeping
A: Well, drying the hair before sleeping on it is a good idea, but not for most of the reasons advocated by your mothers and grandmothers. Going to sleep with damp hair WILL NOT give you pneumonia or any other illness. However, there are some definitive reasons for drying the hair before going to bed.
This only becomes a real concern when the hair is also confined in such a way as to prevent the evaporation of the moisture in the hair. Therefore avoid wearing caps wrapping the head in scarves for long periods when the hair is damp.
Second: we all know that water breaks the physical side bonds in the hair and these reform as the hair dries. When you go to bed with damp hair, the hair is often maneuvered into strange configurations and may dry into these shapes.
This would result in tangles and snarls and could result in breakage. Even if you gather the hair into a ponytail, you risk creating hard bends in the hair that have to be smoothed out with extra styling efforts.
Finally, there is the matter if the hair is damp and kept in a warm (body temperature) environment without being able to become dry. You could develop an odor that's unpleasant. Of course, this becomes less likely with freshly shampooed hair, but the skin cells and other dander that can be found on pillows and in bedding could contain bacteria that would grow wonderfully in the moist, warm environment that would exist as your damp head rests nestled in the pillow.
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See also:
Hair damage
Why wet hair is more fragile
Why you should not brush your hair when it is wet
How to protect your hair during the night, when sleeping