Really tired people can fall asleep almost anywhere, as William Shakespeare observed:
Weariness
Can snore upon the flint, when resty sloth
Finds the down pillow hard.
If you are able to lie on a hard floor and go to sleep immediately during the middle of the day, you are probably sleep-deprived. (An obvious point, but no less true for that.)
Sleep will eventually force itself upon you if you go long enough without it. When scientists carry out sleep-deprivation experiments with human volunteers they often have to work quite hard to keep their subjects awake. Someone who has been deprived of sleep for two or three days requires stimulation, activity and variety to stop them falling asleep, no matter how well motivated they are to stay awake. Ian Oswald, a British scientist who conducted sleep-deprivation experiments in the 1960s, recalled walking the streets of Edinburgh flanked by exhausted volunteers whom he was working hard to keep awake. Even then, he would often see their eyelids closing as they walked along in a twilight state somewhere between waking and sleep.
Sleepiness and its flip side wakefulness determine how alive you feel and how well you perform every day of your life. But until the late 1970s there was no standard tool for measuring sleepiness. Then Mary Carskadon of Brown University in the USA invented a simple technique called the multiple sleep latency test (MSLT). It measures sleepiness like this. The subject is invited to go into a dark, quiet room in a sleep laboratory, to relax and fall asleep. Sensors detect the physiological signs of sleep (of which more later). As soon as the subject falls asleep, he or she is woken up. The key measurement is the time taken to fall asleep. The purpose of the MSLT is to measure daytime sleepiness, so the tests are generally carried out between ten in the morning and eight in the evening. The procedure is usually repeated four or five times during the course of a day.
The length of time between lying down and falling asleep is called the sleep latency. A daytime sleep latency of 15–25 minutes is generally interpreted as a reassuring sign of normality. Someone who is sleep-deprived will fall asleep faster. A sleep latency of less than ten minutes raises questions; there is a good chance that the person concerned will be experiencing intrusive daytime sleepiness, especially during the normal afternoon dip in wakefulness or when bored. A daytime sleep latency of less than five minutes usually signifies excessive tiredness, implying that the individual is substantially sleep-deprived or suffering from a sleep disorder.
Not everyone who falls asleep quickly in the MSLT is sleep-deprived, however. There are a few healthy people who can fall asleep in a trice, given the opportunity and the will. These individuals have the ability to go out like a light even after they have had unlimited amounts of sleep and without displaying any other signs of sleep deprivation. They just seem to be unusually good at relaxing and going to sleep during the day, regardless of their need for sleep. An unusually short sleep latency should therefore be viewed as strong evidence of sleep deprivation, but not conclusive proof.
The MSLT is conducted in a sleep laboratory, using electronic sensors to determine the onset of sleep objectively. There is, however, a crude, do-it-yourself version of the MSLT. This entails lying on a bed under sleep-friendly conditions – lights off, shoes off, TV off, no noises off, and so on – with one hand dangling over the edge of the bed and holding a metal object such as a spoon. Under the dangling hand rests a plate or some other hard surface. When you fall asleep your muscles relax and the spoon drops onto the plate with a loud clatter. All being well, you should then wake up, check your stopwatch and write down your sleep latency. If, on the other hand, you wake up several hours later to find your spoon lying on the plate, it may be that you are sleep-deprived. An even simpler way of assessing your own subjective sense of sleepiness is with a self-rating method called the Stanford Sleepiness Scale. This is a seven-point scale, ranging from 1 (defined as feeling active, vital, alert, wide awake), through 2 (functioning at a high level, not at peak), 3 (relaxed, not full alertness, responsive), 4 (a little foggy, not at peak, let down), 5 (tired, losing interest, slowed down), and 6 (drowsy, prefer to be lying down), to 7 (almost in a reverie, hard to stay awake).
Sleepiness is affected by many other factors besides the length of time since you last slept. Foremost among these is the time of day. Other things being equal, it is easier to fall asleep at certain times of the day and harder at other times. Wakefulness fluctuates naturally over the 24-hour cycle, with low points occurring in the early hours of the morning and again in the afternoon. This daily rhythm in wakefulness is mirrored by fluctuations in performance, which manifest themselves in many ways. For instance, you are more likely to crash your car or make a mistake when performing a difficult task in the early hours of the morning or in the afternoon. Sleep deprivation amplifies the effects of time of day, so the more sleep-deprived someone becomes, the deeper their daily troughs in wakefulness.
Stimulation and activity can mask sleepiness, at least for a while. Busy people often fail to notice how tired they really are until they relax at the weekend or on holiday. Their true state then becomes apparent. Sleepiness is even influenced to some extent by the nature of your previous activity. Experiments have confirmed, for example, that people feel sleepier and fall asleep more quickly after watching television than after taking a walk.
Thou art inclined to sleep; ’tis a good dullness,
And give it way: I know thou canst not choose.
William Shakespeare, The Tempest (1611)
What does it feel like to be seriously tired? For most of us real exhaustion is thankfully a rare experience. But for some, it is all too real. One of the most graphic accounts ever written about crushing fatigue can be found in the autobiography of the American aviator Charles Lindbergh. In 1927 Lindbergh made the first solo non-stop flight across the Atlantic in his single-engine plane The Spirit of St Louis. It lasted 33.5 hours. In his 1953 Pulitzer Prize-winning autobiography, Lindbergh described how his flight from New York to Paris came close to disaster – not because of mechanical failure or bad weather, but because of simple lack of sleep.
Before his marathon flight Lindbergh was told that it would be impossible for one man to fly an aeroplane alone for 30 or 40 hours without sleep. Lindbergh disagreed: he had worked for more than 40 hours at a time without sleep, so he did not see why he should be incapable of flying for that long sitting down. He was almost proved wrong.
The timing of Lindbergh’s departure remained uncertain until the last moment. It all depended on the weather. When an opportunity suddenly arose to take off the following morning at daybreak, Lindbergh went back to his hotel to cram in a few hours’ sleep. As a professional pilot he had learned from experience that every little bit of sleep helps. Even so, it was close to midnight before he reached his room, leaving barely three hours if he was to be ready to fly at dawn. As he eventually lay down to sleep, Lindbergh regretted the flawed planning that would force him to set off in a state of sleep deprivation. He knew he would have been better off starting his marathon fully rested – not just because that would make him a better pilot, but also because tiredness would prevent him from fully appreciating the experience. Lindbergh wanted to enjoy his flight. Even then, in those precious few hours, his sleep was further eroded when someone woke him to ask a stupid question. By three o’clock that morning Lindbergh was at the airfield preparing to take off. And so began his epic, sleepless flight across the Atlantic.
After only a few hours in the air, Lindbergh felt tiredness creeping up on him. How pleasant it would be, he mused, to doze off for a few seconds. He shook himself. He could not afford to feel like that so early in the trip. Later that day, and still less than nine hours into the flight, fatigue hit him again:
My
