What is sub space?

The reason sub drop occurs, is a direct result of sub space... as you are taking your submissive deeper and deeper into sub space, the brain is producing more and more endorphins, and adrenaline... so much so that it goes into overdrive, and produces far more than the body's normal and natural amount, hence the euphoric high the submissive experiences.

After your session is over, the brain takes a while to realise there are no more external stimuli present and then begins shutting down production of those chemicals in order to attempt to bring the chemical levels back down to a level state. What it does however is it completely shuts down production... so much so that the levels often drop below normal after the fact, usually by the next day. 

When this happens, there is usually a period of depression associated with lower than normal chemical levels in the brain, then as the brain realizes the levels are low, it begins production again, but over produces once more to compensate, not nearly as much as when in a scene, but still, which leads to a period of happiness and giddiness once more. This is kind of a roller coaster, produce too much chemicals, then shut down, level drops bellow normal, overproduce again, and its a roller coaster effect as this goes on until the brain finally gets to a normal state again. This period of depression and euphoria, then depression again and so on is sub drop, it usually leaves the submissive feeling very confused and emotionally unsure, and vulnerable, a period in which she/he relies heavily on her/his Dominant, and for most new submissives a very scary period.

That for the long and short of it is the mental reason for sub drop... it has nothing to do with true depression, it has nothing to do with the submissive disliking or being mad with the Dominant, it has to do with chemicals, plain and simple... and it is every Dominant's responsibility to realise that, and plan for it, and to help the submissive work through that period and support her/him. Often I have heard that exercise helps to level out the sub drop effects much quicker, so if you are a submissive, and want to shorten the effects, go for a nice run, or a bike ride or something the day after.
 

~Author Unknown~ .