Americans today are sleeping less (seven hours nightly) compared to nine hours nightly a century ago, according to Harvard Medical School.

Sleep deprivation can be complete or partial, based on duration and severity. Complete sleep deprivation occurs when you stay up all night. Partial sleep deprivation happens when you get less sleep than you need for several days (like getting six hours of sleep for two weeks), but the mental and physical effects are the same.

When sleep-deprived, simple tasks become difficult and can even be as debilitating as intoxication. One study concluded that 24 hours of wakefulness caused the same impairment as a blood alcohol content of 0.10 percent. Sleep deprivation also plays a role in thousands of automobile accidents annually.

Partial sleep deprivation occurs when you get some sleep, but not 100 percent of what is needed, thereby building up a sleep debt. After a single night of short sleep, you might not notice anything amiss. After two or more nights of short sleep, people are often irritable and work performance declines.

Long-term partial sleep deprivation occurs when you get less than the optimal amount of sleep for months or years, which is common for insomniacs.

Even healthy people have this problem. A group of student volunteers who slept four hours nightly for six consecutive days developed higher blood pressure and higher levels of cotisol, a stress hormone. They also produced only half the number of antibodies to a flu vaccine and showed signs of insulin resistance, a precursor to type 2 diabetes. All changes were reversed when the students caught up with the hours of lost sleep.

A growing number of studies link long-term sleep deficits with significant health problems. Researchers tracked the sleep habits of 153 men and women for two weeks, then quarantined them for five days and exposed them to cold viruses. Those who averaged less than seven hours nightly were three times as likely to get sick than those who averaged eight hours or more.