If you are sleeping in the wrong kind of bra, then you could really run into some serious problems. It can impair the blood flow and lymphatic drainage, which, at worst, can lead to chronic inflammation, oedema (fluid retention) and discomfort. If you find marks on your body from your bra strap or the side panel of your bra, then you might be wearing a bra that is too tight and constrictive for you.It's also important to consider the material and make of the bra that you are sleeping in. Bras with underwire can push on your chest when you are laying down and this can result in cysts over time. Something a little softer, like a sports-bra, is much better. Most underwire bras are made of a synthetic polyester material. This is a material to be wary of when choosing the right bra to sleep in because they can raise the temperature of your breasts.Your breasts are external organs that meant to work best at a lower temperature than the rest of your body. There isn't a ton of research to back this up yet but there are some theories that this may cause breast cancer. Look for a stretchy bra that's ninety eight percent cotton. So, here's my advice: it's OK to wear a bra to sleep if you feel like it. Just make sure that it is the right one.