productivity without the dreaded caffeine crash. From Memory Foam to Hybrids: The Best Mattresses You Can Buy According to Our Experts See all photos Why we feel sleepy in the afternoon The natural dip in energy and focus that you feel after lunchtime is known as the "afternoon slump." This is part of your circadian rhythm, the biological clock inside your body that regulates your sleep cycle. It's caused by fluctuations in hormones and neurotransmitters, particularly cortisol and adenosine. Cortisol makes you feel awake and alert. Cortisol levels in the body are generally higher when you first wake up and decrease steadily throughout the day. However, your body produces more cortisol in response to certain stimuli, such as exercise, which is why an afternoon workout can make you feel more awake. Adenosine, on the other hand, makes you feel sleepy, and your body secretes more of it as the day goes on. (Fun fact: Caffeine blocks adenosine receptors on your cells, which is why it works to keep you alert.) Aside from your natural body clock, things like nighttime sleep quality, diet, caffeine consumption, room temperature, screen time and exercise habits influence afternoon fatigue. Sleep disorders, such as sleep apnea and insomnia, also contribute to daytime sleepiness. Who shouldn't take a nap Dr. Ramiz Fargo, medical director for the Loma Linda University Sleep Disorder Center, told CNET that most people can take naps and still enjoy a healthy sleep cycle, but people who struggle with insomnia should avoid napping. In people who already experience nighttime wakefulness, napping can exacerbate the problem and lead to a sleep detriment in a few ways, including: If you aren't diagnosed with a sleep disorder and you don't usually have trouble falling asleep or staying asleep at night, you can most likely nap without issue.