Throughout a very mentally taxing day of learning new and important things, the hippocampus becomes overtaxed, as it acts as a short-term memory reservoir. This can make one feel exhausted. During deep NREM sleep, a wave of electrical activity in the brain provides a path between the hippocampus and the cortex, which is the place in the brain we put our long-term memories.
Each night, before we go to sleep, we have an opportunity to bring to the surface those things during the day that we would like to remember. This is why journaling and learning a language is encouraged before bedtime. We have the power and control of what enters our hippocampus in the evening before bedtime. As long as we have a successful deep, NREM sleep (usually occurring within the first few hours after falling asleep), the short-term reservoir will get dumped into the long-term reservoir of the cortex.
During a day in which a lot must be studied and learned, a long daytime nap following a morning study session will allow a significant 20% learning advantage in the late afternoon and early evening learning hours to those who don’t take a nap. Even when those not taking a nap take a long break of some mindless activity.
If you find this information interesting and would like to learn more, I found it in the book Why We Sleep: Unlocking the Power of Sleep and Dreams by Matthew Walker, PhD.