Some people have difficulty staying awake throughout the day without taking a nap. Some people have difficulty staying asleep throughout the night. Often there is a connection between the two.

For people with insomnia, the most common symptom is difficulty staying asleep, also known as sleep maintenance insomnia. It may be that they wake up frequently during the night or wake up only once but then can't return to sleep for hours. For someone expecting to be able to sleep through the night, this can be extremely frustrating.

Not getting enough sleep during the nighttime may in turn lead to daytime sleepiness and the temptation to nap. This can become a vicious circle if the napping then interferes with the ability to get enough sleep the following night.

Of course, some people make daytime napping a part of their daily routine, especially when they stay up relatively late in the evening and get less total sleep at nighttime. This type of sleep schedule is perfectly normal in the cultures where the after-lunch siesta is commonplace. As long as people are getting a reasonable amount of sleep with their combined day and night hours, they should be fine during their waking hours.

If we get plenty of sleep during the nighttime, then usually we can cruise through the day fully alert. All of us, however, have a biological tendency to be a little sleepier in the early-to mid-afternoon. And if we're deprived of sleep to begin with, we'll be more likely to feel sleepy in the afternoon, and perhaps even succumb to a nap.

Some believe that the tendency to awaken in the middle of the night might be the physiological opposite of the tendency to experience sleepiness in the afternoon. This entire 24-hour cycle — with some afternoon sleepiness and some middle-of-the-night sleeplessness — may be an inherent feature of the biological rhythms that regulate our sleep and waking mechanisms.

In some cultures, the people will customarily sleep for a few hours, be awake for a few hours, and then return to sleep again for a few hours. Sometimes these two sleep episodes are described as "first sleep" and "second sleep," and these folks don't see it as a problem, but rather as an expectation. Some research studies have even shown that most people, if they're kept in dim light for 14 straight hours through the evening and nighttime, tend to have these two separate episodes of sleep.

Insomnia becomes a problem when people are unable to sleep when they expect to sleep and when they suffer undesirable consequences because of their inadequate sleep time. If that's you, then talk with your doctor about it. You might have a sleep disorder, for which lots of treatment options are available.