Sleep is more or less essential to ensure that our bodies and minds function, assuming that one has a somewhat high standard of the definition of the term 'function'. It is, however, possible for one to function after having gone for extended periods of time without sleep, provided they lower their standards.
Symptoms of Sleep Deprivation
The first things you are most likely to notice, if you are sleep-deprived, is a drastic increase in the frequency of your yawns. Now yawning, contrary to popular belief, is not brought on by lack of sleep but instead by a decrease in the amount of oxygen that gets to the brain. This decrease in the flow of oxygen usually happens when one doesn’t take deep enough breaths which in turn, is usually caused by lack of sleep.
The second thing you may notice, is a severe decrease in proper brain function. The definition of 'proper' is debatable, but regardless of how well one's brain performs when you've slept adequately, there will, no doubt, be a noticeable decrease in performance as one's sleep level goes down. Sleep deprivation is one of the most common causes of stupidity in mildly intelligent people.
Another thing you may notice is a difficulty in exerting yourself physically. This often shows itself in the form of an unwillingness to stand up after having been sitting or lying down for extended periods of time, or sometimes in the form of a small grunt, let out while lifting a small, light object such as a spoon or a television remote. Carrying heavy objects become impossible, as does gripping things for long periods of time, and running marathons are just plain out of the question. All round lethargy is an accurate description of this condition.
What to Do if you are Sleep Deprived
The most obvious answer can be stated in one word: sleep.
Since the above answer is not always possible, people often find other solutions as to what to do when they've not had enough sleep.
One popular choice, when one notices a symptom is to ignore it. This often works for short periods of time, but inevitably leads to an even more severe case of sleep deprivation.
Another very common solution that is often applied when the first two have failed, is to take drugs. Drugs like caffeine tend to diminish the effects of sleep deprivation. Caffeine is often found in coffee, a dark brown drink that is often palatable when mixed with milk and loads of sugar. Unfortunately, caffeine has addictive properties and often leaves people unable to function in any way until they have had their morning cup of coffee.
Still, the best thing to do if you find yourself unable to function after having gone for many, long hours without sleep, is to cancel your appointments, find a nice cosy place to lie down on a couch, bed or under a dumpster, shut your eyes and go to sleep.