Question: When you have dreams or nightmares , why is this? Is it something to do with what you've experienced before or something in your mind? I once learnt it was something to do with your ancestors ?!

Nancy Carlisle answered on 17 Jun 2014:

I’m not sure about all our experiences when we are asleep, but there is evidence from recordings taken in rats that when we are asleep we re-live experiences from the day.
Here’s how they know: Certain cells in our hippocampus will activate only when we are in a certain location (i.e. walking down the hallway to the bathroom, entering the bathroom). You can record from these cells when rats walk a certain path. Then, later, you can record from these same cells when rats are asleep. Interestingly,when two cells respond/ signaled together during the paths the rats did that day, they were more likely to respond together during sleep too. This suggests that the same kinds of activations that go on during our daytime activity will get ‘replayed’ at night!