We left off talking about the circadian rhythm and how it is primarily regulated by light during the day, helping us be more alert, and conversely by melatonin at night, making us feel sleepy. This can actually help define what type of person you are- either a lark (those who are always “bright-eyed and bushy-tailed” first thing in the morning), or an owl (people who “do their best work after 10 PM”). It also goes along way toward explaining a major difference between teenagers and older people.
We all know that teenagers have a tendency to go to sleep very late (1-2 AM or later) because they complain that they’re not tired. This is because their circadian clock is “delayed” or shifted to a later hour. They actually don’t get a melatonin surge until much later than a typical adult. This is also the reason that they can’t get up in the morning. It’s not (just) because they are being lazy. Their body clock is set so that they can sleep very well between the hours of 3 AM and 11 AM. Also note that teens probably do need more sleep than the typical adult, about 9 hours compared to the normal 7-8 hours a night.
Now think about older people, in their late seventies or eighties. They want to go to sleep by 7 or 8 PM, and then they wake at 4 AM and are no longer able to sleep. This is because their circadian clock is “advanced” or set earlier than the mainstream.
Getting back to the topic at hand. The circadian rhythm functions at all times, but due to its influence on metabolism and hormonal levels there are certain times of the day that the sleep drive is running at its highest. This generally occurs between the hours of 2 AM and 4 AM, when we are usually trying to sleep anyway, but also during the afternoon between approximately 1 and 3 AM. The more you have slept previously the less you are susceptible to it, but it is actually a normal response to feel sleepy in the early afternoon. Some suggestions that may improve that feeling are exposure to the sun and possibly a well-timed Starbucks coffee in the late morning or early afternoon.
On the other hand, there are times that despite many hours of sleep deprivation, let’s say after an “all-nighter,” that it is still difficult to fall asleep, despite a strong homeostatic sleep need. Since your body may be accustomed to being up and about at 7 or 8 AM, not settling down to sleep, it often can be more difficult to fall and stay asleep.
I hope this helps clarify the issues that were raised by Emily in her question. But let me remind you that there is always the possibility of a sleep disorder. Read my SharePost, “How Do You Know If You Have A Sleep Disorder?” and discuss any issues with your doctor.