After the long holiday, many night owls find themselves trapped in a vicious cycle: They are unable to catch up on sleep during the day, despite feeling exhausted, and yet they struggle to fall asleep at night. But how exactly does the body distinguish between day and night, and how does it know when we've been staying up late?
"The core mechanism for telling day from night lies in the synergy between the biological clock and external signals," said Dr. Li Mao, associate chief physician in the Neurology Department of the First Medical Center of PLA General Hospital. The human brain houses a command center called the suprachiasmatic nucleus (SCN), which functions like a precise timer. It receives light signals from the retina and uses them to regulate bodily functions.
During the day, intense light transmitted through the retina suppresses melatonin secretion, keeping the brain alert and enabling efficient daily activities. As night falls, the SCN lifts its inhibition on melatonin secretion, triggering a significant increase in its production and thereby signaling to the body that it's time to sleep.
"Simultaneously, multiple physiological activities synchronize with the day-night cycle, collectively reinforcing the circadian rhythm," Dr. Li said.
The body's response to staying up late essentially reflects disruptions to the circadian rhythm. Even when we force ourselves to stay awake at night, the suprachiasmatic nucleus in the brain remains vigilant, issuing rest signals according to the normal rhythm and increasing melatonin levels as scheduled.
"At this point, signals such as drowsiness, decreased attention and slower reactions are reminders that the body needs rest," Dr. Li said.
Moreover, staying up late disrupts the body's hormonal balance. For example, cortisol helps to maintain alertness during the day, but it should decrease gradually at night in order to prepare the body for sleep. However, late nights cause cortisol levels to spike abnormally. Over time, this exacerbates fatigue, creating a vicious cycle whereby the longer you stay up, the more exhausted you feel, and the harder it becomes to sleep.
Dr. Li cautions that the impact of staying up late extends to multiple areas. The effects include reduced metabolic efficiency, disrupted neural repair processes and weakened immune function. "These physiological abnormalities clearly signal to the body that its designated sleep period is being usurped," he added.