"Sub Drop can come in many different forms. Sub Drop is the emotional and physical affects of the release and drop of endorphins in the body after a play session. Most of what you read online are the physical aspects; the fatigue, sadness, aches and pains and recovery from marks. There is a more intense side of Sub Drop that gets very little attention because for each person it is different and describing how to recover can take many forms.
If not cared for, you could go into depression just from one play session. The endorphins and other hormones released during play leave your body in such a way that it takes time to rebuild the balance of hormones in your system. You could feel like you have a hang over or partied too hard the night before, you could feel lost and depressed for hours or days. You may just want to sleep it off. These are the more extreme forms of Drop. Some people recover in a matter of hours, but others could exhibit signs of Sub Drop for weeks after an intense session."
See the entire article @ Submissive Guide