we all have periods of high energy and low energy in the same day. paying attention to this allows us to focus getting the things we need to get done during our periods of higher energy. being depressed saps us of our motivation strenght and energy. so you may start off strong, but it takes a lot of energy to make it thru the day. by five, you just dont have anything left anymroe. this makes total sense. maybe if you took a little time in the middle of the day to relax and rejuvenate you would be able to make it longer into the evening.