A figure sits by a campfire outside the entrance to a cave, huddled in layers of warm furs. The wind carries the chill of the glaciers that loom across the northern horizon as far as the eye can see. The sun is starting to sink toward the west, and the sentry’s eyelids are getting heavy. Around him, his extended family drifts into the cave, ready to get a good night’s sleep. He longs to join them. Soon, now…

He is in the middle of a vigorous yawn when his brother approaches with a broad smile and a spring in his step. While the rest of the group had risen with the dawn to attend to the business of hunting and gathering, his brother had slept until the sun was at its zenith in the sky. The sentry doesn’t understand why his brother seems to live by a different rhythm, but he is grateful to know that a fully alert person will be up all night to keep beasts from attacking the family while they slept.

We all have an innate sense of time that tells us when to eat, sleep, and wake: our circadian rhythm. Some people are night owls and others prefer mornings, but for the most part, we follow the sun’s cues for when to get up and when to go to sleep—give or take a few hours. In a small segment of the population, however, this rhythm is completely different. One of these circadian rhythm disorders is Delayed Sleep-Wake Phase Disorder, also known as Delayed Sleep Phase Syndrome. I prefer the DSPS label, as this variation is only a “disorder” in the context of civilization after the Industrial Revolution. Before electric lights and alarm clocks, it was a great asset for a community to have a member or two who were at their best during the hours most people were asleep. Then the modern “nine to five” routine began to take over, natural deviations in circadian rhythms were stigmatized, and people were forced to conform—often to the detriment of their mental and physical health.

People adjusting to a new time zone or suffering from insomnia may benefit from light treatments, melatonin supplements, meditation, and similar measures, but for many DSPS sufferers, these methods only cause small, short-term changes. Advice like “just go to bed earlier” or “tough it out for a while, you’ll adjust” is not helpful to DSPS sufferers, any more than “just walk it off” is helpful to someone with a broken leg. If such common-sense measures worked for them, they wouldn’t have DSPS. It’s not a matter of laziness, either: getting a consistent eight hours of sleep should be considered normal, but many scoff when the start and end times are, say, 3 A.M. to 11 A.M.

When you think about it, it’s odd to impose a moral value on the hours that others sleep. Perhaps it goes along with the idea that crime and other immoral acts tend to happen under the cover of darkness, so anyone who is active after sunset must be up to something nefarious. Because daylight is crucial to strenuous activities like agriculture and construction, people engaged in those pursuits tend to get started as early as possible, leading us to associate early risers with hard work.

To most people, a 9 A.M. appointment is no big deal. To DSPS sufferers, it may be physically impossible, or at least a sickening ordeal whose effects linger for the entire day. Everyone is different, and some can function on a few hours of sleep if needed, but it’s not healthy. The Mayo Clinic recommends at least seven hours of sleep a night for most adults. Sleep deprivation is proven to lead to a host of mental and physical health problems. Name a condition, and chances are good sleep deprivation can cause it or make it worse: heart attacks, heart disease, strokes, weight gain, reduced immune response, depression, anxiety, memory loss, high cholesterol, headaches, difficulty concentrating, and more. The rates of car crashes, medical errors, and industrial accidents all increase when people have insufficient sleep.