The "three good things" exercise was developed by psychologist Martin Seligman and colleagues. Here's how it works, according to UC Berkeley's Greater Good Science Center:
Every night before bed, write down three things that went well for you, along with an explanation of why they went well. The good things can be as seemingly small as your partner taking out the garbage or as big as getting a promotion. Make sure you include as much detail as possible, as well as how the event made you feel.
Seligman and colleagues found that people who used the three good things exercise felt happier and less depressed for six months.
Again, you can do the same exercise in the morning, if that works better for you.