When I was younger I worked nights and between 3am-5am mostof us would dip, our mood lower, lethargic and someone said those hours are when your body is at its weakest
Please please don't be alarmed but for sick/terminal/elderly patients it's a common time of night to pass away (seriously I'm not suggesting anything just that maybe it's because their bodies have hit a low)
Anyway back to my night shift days, by the time we'd clock off at 6am we were brighter and mood had lifted x