This is a truth for any illness or disease. If you had surgery you would need lots of rest to heal. It's the same with depression and anxiety. Getting thru the day when you have these illnesses can be especially difficult. You need more energy and focus to make it thru the day, at least I do to stay happy and healthy. Getting a good nights sleep is essential to recovery. I get a lot of information from WebMd. Here's a great article on this very topic. http://www.webmd.com/depression/guide/depression-sleep-disorder
This explains it all. I want to cover this now to help you on your way to recovery or to simply remind you of this fact.