The big question is: why is the idling brain so active? There are plenty of theories, but no agreement yet. Maybe different brain areas are simply practising working together. Perhaps the brain is staying active like an idling car, just in case it needs to act suddenly. But it’s possible that those mind wanderings and replays of our day play a vital role in helping us to consolidate our memories. We know that our dreams seem to play a part in sorting out our memories – now there is evidence that it happens during the day too (in rats, at least).

We also know that when the mind is left to wander, it often focuses on the future. We start thinking about what we’re going to eat in the evening or where we’re going to go next week. All three of the chief areas of the brain involved in imagining the future are part of the default mode network. It is almost as though our brain is programmed to contemplate the future whenever it finds itself unoccupied.

Moshe Bar from Harvard Medical School thinks there might be a very good reason for that. He believes daydreaming essentially creates memories of events that haven’t happened. This gives us a strange set of “prior experiences” we can draw on to help us decide how to act if the daydreams ever do come to pass. For instance, many air travellers have wondered what it might be like to crash. Bar’s idea is that if the plane did actually crash, the memories of all those daydreams from previous flights would come into play and help the passenger decide how to behave.