Yes ... lethargic is how I describe the feeling of emptiness, no emotions, and having no energy.
The feeling started kicking in at around 30 days into a hard reboot.
At my worst, before rebooting, I would regularly PMO daily, only taking a "day off" so to speak, to recuperate. It became a habit ... and my brain was "programmed" to expect the next "session".
When the reboot first started, it was a feeling of relief to be free from the pressure of having to satisfy the yearning from the brain.
But for me, it's about 30 days into each reboot, after my brain had gone from a state of "hyper activity" ... to suddenly getting no stimulation, that the feelings you describe kick in. It's as if my brain can't settle down.
Stick with the reboot. Don't give in. Too many times I have struggled to get pass this stage. It will be worth it.
... and so onwards we go!