Depends on the night.
For a physical challenge there have been numerous times I can remember where I am gasping for wind and needing some sort of quick break....A new uki is always called at the right time
. Other times I go the whole class without feeling tired at all (Still learning quite a bit just not in the physical manner).
Mental challenge happens all the time. I always feel like "Hey, good, you're doing that sort of right, now try this!" which always kinda cool. Everyonce in a while I hit those night where all it feels like I'm getting a bit lump in the shoulder from frustation... no matter how bad those nights are I always like them the next morning as I slowly start understand what I might have been doing wrong. Which makes for a great day at work