Sleepiness is a part of the daily circadian rhythm. One would not feeling sleepy all day or for a week.

One can feel tired without being sleepy. The purpose of getting up at the same time is it helps the body set that sleep clock for sleepiness to set in 14 to 16 hours later. Sleepiness is triggered by awake time.

The mental fatigue of PCS is an entirely different issue. But, it can be managed so it does not interfere with the sleep cycle. A very short nap or doze can reduce mental fatigue without messing up the sleep cycle by much. 5 to 10 minutes should be the max.

Mental fatigue is often the result of lack of qualify of sleep. For me, it was a result of sleep apnea that also left me tired during the day. I would struggle to think because my brain was still full of the toxins from previous days.

A common problem with sleep is having food energy to use during sleep. Some people have a blood sugar drop 7 hours or so after a meal. This can interrupt sleep. I have to have 150 to 200 calories before bed. A slice of cheese with some crackers, some raisins and nuts, a piece of toast with peanut butter on it. I have a snack and 30 to 45 minutes later, I am ready for sleep and I will sleep soundly for 6 to 7 hours.