This is a very common question and there are two opposing schools of thought on the issue. Those who say you should not wear them overnight generally justify that claim by saying that the supporting gums under the denture need massaging so that blood flow is encouraged, thereby maintaining the general health of the gums. They also claim that there may be a chance of the denture becoming tangled up in your mouth if it becomes dislodged. Those who claim you should leave dentures in offer many more reasons for you to do so. They claim that the general movement of the denture upon your gums during eating and talking gives enough massage for blood flow.
Most dentures are, or at least should be, secure in your mouth so dislodgement should rarely be an issue. The most important reason offered for leaving dentures in overnight though is to reduce the risk of damage to nerves in the joint that supports the jaw. That joint is understood to be one of the most complex in our body and has numerous nerve endings that could possibly be damaged if the jaws closed too far. Simple overclosure of the jaws without the dentures in place could cause damage to those nerves because the jaw is designed to only close to a certain point. When we are asleep, we have no control over the possibility we could be causing damage, so it is generally safer to sleep with dentures, especially if there are no remaining natural teeth.