- Softness: Covering hair up for a while helps to soften it. I do it for about 15 to 30 minutes (after spritzing with water) before styling when my hair is very dry or matted. It does work.
- Prolonged softness: Some argue that having a shower cap over their head overnight once a week helps to keep it soft for the rest of the week.
- Retention: Soft hair is less prone to breakage and therefore allows hair to grow longer.
- Friction: unless the hair is well oiled or has conditioner on it there will be friction between the hair and the shower cap possibly causing breakage.
- Scalp doesn't breathe: if you have a bag over your head you're preventing the pores on the scalp from breathing freely.
I think having a bag over your head overnight once a week will probably do no harm, however, I think if it's done daily it could impact hair growth due to the delivery of oxygen to the scalp. Like I say, there have been no scientific studies to explain whether or not it's good but for the moment it's not something I plan to do on the regular.
Even when my hair is very oily it doesn't stain my sheets, it stays within the satin bonnet so this is not even a consideration for me. Wetting my hair and baggying for 30 minutes does the trick so I'm reluctant to do it overnight for fear that my hair will go rancid.
I like the idea of baggying the ends only; I might add this to my regimen...