Quoting lesswrong.com:

Even if I have a lot on my plate, being happier will help me do it. It’s like sleep: it’s easy to keep staying up and staying up, because sleep just seems so unproductive, and you can get some work done however tired you are. But over the long term, getting to sleep at a sane hour every day will let you accomplish more; and so with maintaining a good affect consistently. Mood maintenance is typically not the most immediately productive thing I could be doing, but treating it as my top priority save in dire emergency has let me be more effective than I was before.