I lift weight in the morning (1h routine) then I take a shower and rest for 30 to 60 minutes. I usually feel my body tired, almost exhausted. But, my brain feel

I lift weight in the morning (1h routine) then I take a shower and rest for 30 to 60 minutes. I usually feel my body tired, almost exhausted. But, my brain feels relaxed, light, clean (not sure how to explain it). I have no idea why it happens this way but it's been working for years.