Sometimes curious about this, usually because I forgot to note what time I arrived at the office, and I keep forgetting how to check. So, here’s some commands I found on SuperUser which seem to do the trick:

Also discovered this one, which will create a more thorough report, which can be interesting to look at:

Just discovered a very simple way to find what’s keeping my Windows computer awake or my screen from going black.

1. Open up an elevated command prompt.
2. Run the following command
   > powercfg -requests

While playing a video in Windows Media Player I get this output:

Simple and clear!