I've recently been reviewing some of the notes I have taken while reading various articles. Here's a note that jumped off the page; studies suggest that during sleep your brain is flushing out the waste generated by neural activity.

In 2013, researchers Xie et al. published a paper in Science showing that in mice, certain types of brain cell shrink during sleep, which increases the space between cells and allows more fluid to flow through and past the cells. This increase in interstitial space allows more fluid to flow through the brain, which increases the rate at which waste is cleared and carried away - at least, that's what I took away from reading the BBC summary of the paper. In lay-mans terms - your brain is effectively cleaning itself while you sleep.

Here's a verbatim note from Learning How to Learn, one of the most popular ever MOOCs on Coursera, explaining the importance of sleep for memory/retention:

It seems that during sleep your brain tidies up ideas and concepts your thinking about and learning. It erases the less important parts of memories and simultaneously strengthens areas that you need or want to remember.

As if the importance of sleep wasn't clear enough, James Clear tweeted a reminder about the importance of sleep earlier this year:

The best productivity hack is getting 8-9 hours of sleep every night. Second best is exercising 30-60 minutes each day. Both are obvious and overlooked, and yet make a more meaningful and immediate impact on the quality of your thinking than 99 percent of productivity tips.