What Happens During Sleep?

At this second I am thinking about my life. For some reason I woke up very depressed today. I don't know if it was due to Rex not being home and missing him, turning 50 in a few weeks, thinking about money, or something else. What happens during sleep that makes us wake up happy, depressed, moody, etc? I know that I was dreaming so much, but I don't remember them and I did wake up many times during the night. I know that when I am feeling this way, I try to be positive and think about all the good in my life. It is amazing how much you really have to be thankful for and how that can help you get through the day. Sometimes, I am not happy with where my life is at this moment. However, looking back, I have had many accomplishments and I just need to take all my successes and everything I have learned and use it towards my next venture in life.