Circadian rhythms dictate how bodily functions ebb and flow over a roughly daylong cycle, experts said, and these rhythms are largely set by a biological “clock” in the brain. Sleep pressure, meanwhile, is the idea that our need to sleep gradually builds the longer we’re awake.
Our circadian rhythms are influenced by the environment, which means we generally feel awake and alert during daylight, and sleepier when it’s dark. But some fluctuations are normal: For instance, people tend to feel quite awake in the late morning, said Dr. Alon Avidan, a professor of neurology at the University of California, Los Angeles, but experience a dip in alertness in the afternoon — when certain neurotransmitters linked to feeling awake may become less active.
Sleep pressure is generally lowest when we wake up in the morning — the body is like “a battery that is fully charged from getting a good night’s sleep,” Dr. Avidan said. But the longer we’re awake, the more our energy drains and the stronger sleep pressure becomes, he added.
During the afternoon slump, those two forces — sleep pressure and circadian rhythms — are essentially “fighting each other,” said Dr. Ravi Allada, executive director of the Michigan Neuroscience Institute at the University of Michigan. When we feel drowsy, the pressure to sleep is winning the fight, he said.
How long this feeling lasts can vary, but people tend to get a “second wind” in the early evening, Dr. Avidan said. The dip may feel worse, however, if you’re sleep-deprived or have an irregular bedtime schedule, said David Earnest, a professor of neuroscience and experimental therapeutics at Texas A&M University. Sleep disorders including insomnia or sleep apnea can exacerbate the feeling, too, he said.
#Deal #Afternoon #Fatigue
