My scores at stuff like Lumosity and Quantified Mind tend to always be great in the morning (a couple hours after I wake up), but unless I slept enough the previous night they suck balls in the early afternoon; they get better again in the evening if I take a nap in the afternoon. My self-perceived wakefulness and willpower levels vary similarly, but several hours earlier (i.e. I underestimate my mental performance right before lunch and overestimate it right after my afternoon nap).