Many of us look upon sleep as a huge, boring waste of time, but nothing could be further from the truth: your sleep time is when your brain is literally rebuilding itself and is critical for memory and neurotransmitter levels. I would argue that Sleep, Exercise and Diet: are your greatest and truest friends.
During the REM cycles of the night, the body replenishes badly needed stores of neurotransitters. Particularly important is the rebuilding of seratonin and norepinephrine, which are critical neurotransmitters used for learning and retention that are normally depleted during the day.  In fact, during REM sleep the cells that use seratonin and norepinephrine actually become inactive, allowing the brain to more effectively rebuild itself. Again, one secret to brain power is quality of sleep.