Most people have a 24-hour circadian rhythm that is largely driven by light and darkness. Every 24 hours, you make a full cycle, going from being awake, to sleepy, to awake again. And unless you force your circadian rhythm to move earlier or later (like if you are a shift worker), it usually syncs with daylight and nighttime. But this isn't true for everyone.
If you have Non-24-Hour Sleep-Wake Disorder (a.k.a. "Non-24"), your circadian rhythm isn’t always matched up with daytime and nighttime because your cycle is longer than 24 hours. As a result, you go from being on the same sleep cycle as everyone else to slowly shifting your sleepiness later and later in the day. Eventually, you become most tired during the day and least tired at night. This shift continues until you’re back to where you started, but the cycle just keeps going. This disorder is often due to light not reaching a certain part of the brain, which is why it tends to affect people who are totally blind and can’t sense light. Between 50 and 75 percent of people who are totally blind have Non-24, which is between 65,000 and 95,000 Americans.
It can be tough to accurately diagnose Non-24 because the symptoms can look similar to other sleep disorders. One warning sign that you have Non-24 is that you are completely blind and have periods of healthy, high-quality sleep, followed by periods of poor sleep. For a diagnosis, see a sleep specialist. The specialist will likely ask you to track your sleep or will even monitor internal chemicals like melatonin and cortisol to confirm whether Non-24 is present.
There are a few different therapies and treatment options if you get diagnosed with Non-24. If you have some sight, you can use light exposure to get your body back in sync (morning light to move your circadian rhythm earlier, evening light to move it later). Otherwise, a medication called Tasimelteon or melatonin supplements might help.