There is: tiredness. Often, but not always, due to lack of sleep. A nap is the solution.
If you need 8 glasses of water a day to be at peak-performance but for whatever reason only manage to drink 7 in the morning, why not have that final glass sometime during the day? Same with sleep.
In fact it's absurd to think everybody can store energy upfront and power through 9 hours of activity without falling off towards the end.
Naps are natural.
Consider my opinion changed.
I suggest you recalibrate your definition of "normal".