To answer your question, it depends on the braids whether they hurt or help. Extensions, weaves, cornrows, and small braids can hurt our hair because they are often pulled really tight, and then additional weight is usually added, pulling at your hair (and therefore your scalp) even more. This may cause hair loss or even baldness, (see traction alopecia), which may become permanent. Many of these braided styles can also damage our hair because they're often put in while combing through dry hair(which is damaging), and lots of tension is also put on the hair for extended periods of time (stretching the hair is also very damaging).

However, if you gently put your hair in a low braid at night to protect it, with just enough firmness to keep your hair from matting, (but not so much force that it's pulling at your scalp, or has completely stretched out all your curls), then it's fine. This protects your hair at night, and keeps it from tangling against itself and rubbing against the pillow. As long as your hair isn't being pulled tight anywhere, this is fine.

I've been putting my hair up at night in a braid or bun for all 12 or so years, and my hair is very happy.