“When you are sad, you engage mechanisms to make yourself happier in that moment. One thing that you will do is to seek immediate rewards rather than focusing on larger rewards you might get in the future. That bias may cost you in the future.
If you find yourself feeling sad, try to avoid making decisions that trade off the present for the future. In that sad mood, you will probably short-change your future self in order to get something immediately. Instead, try to delay your choice until your mood improves.”