Psychiatrists know that short term sleep loss can improve mood in depressed people. In fact, in those with bipolar disorder, which used to be called manic depression, sleep loss can trigger a manic event. Sleep deprivation is not used anymore as a treatment because it's so difficult to maintain. The effect only lasts until the person sleeps.
Adenosine is the chemical that builds up in our brain the longer we stay awake. The higher the levels, the more sleepy we get. Adenosine levels rapidly diminish with sleep. Researchers think that the build up of adenosine is responsible for the mood elevating effects of sleep loss. This article discusses how researchers studied mice with depression. They gave the mice a chemical that mimics high adenosine levels. The mice slept normally but they showed improved mood and behavior.
To me this is fascinating because it points to a novel way to treat depression. I also wonder if it would be a way to help insomniacs sleep better.