Stealing this graphic that @ohm posted in another thread:
I personally find that I work much better at night, when there are far fewer distractions - less emails, notifications on my phone, tel calls, posts on forums even.
I also think ‘the sleepy brain’ may something to do with it too (where basically, at a level of tiredness you can’t multi-task as easily, meaning you are forced to focus on one thing) - these articles cover it well:
What about you? When do you feel you work best? 
