https://onlinelibrary.wiley.com/doi/10.1111/jsr.12948
When we look at sleep we have two main categories, people who fall asleep late (late chronotype) and people who fall asleep early (early chronotype). The late chronotype is linked to worse sleep quality and therefore worse psychological outcomes. This study wanted to looked at why some people are in the late chronotype group. They found that people in the late chronotype group had increased activity in the part of the brain that causes daydreaming and ruminating thought called the 'default mode network.' The default mode network (DMN) runs between the front of the brain (medial prefrontal cortex (mPFC)) and the back (precuneus) passing through some important areas in the middle of the brain (posterior cingulate cortex (PCC)). The DMN causes daydreaming within our brain and is deactivated when we focus on external things. If we have a high level of activity and connectivity in the DMN it will reduce how hard our brain is working during the day (leading to less sleep pressure) as well as making it harder to fall asleep due to all the random thoughts; both of these make it more difficult to go to sleep. Therefore, people who have a high connectivity in the DMN have a harder time going to sleep.
