The circadian rhythm controls our sleep-wake cycles, and improved sleep directly correlates to improved health. Often times, I hear people describe sleep as an indulgence via phrases such as "money never sleeps" or "sleep is for the weak," when the reality is that huge segments of American society are sleep deprived and therefore operating below optimal levels of physical and mental health. Structured sleep necessitates consideration for one's personal factors as well as an understanding of environmental and cultural influences.
Here's some factors that contribute to typical wakefulness and restfulness.
The above image gives a faint idea of how two pivotal hormones in your brain are controlling your sleep-wake cycle. In addition, you have a hormone called adenosine which gradually rises throughout the day, applying "sleep pressure."
Something I want to emphasize with the graph is the "afternoon slump." You can see, with the decreased cortisol, why many people might feel exhausted by the early afternoon. In addition to societal pressures, we often fail to give ourselves breaks during this "afternoon slump," a perfect time do something that promotes wakefulness. Let me be frank, the answer isn't to increase your cortisol (which would induce too much stress and anxiety), but it's to seek activities that also promote wakefulness:
When you do need to call it a "night," the National Sleep Foundation provides some helpful tips on promoting basic sleep hygiene and sleep habits for better rest.
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Website: National Sleep Foundation