Tired or sleepy?

Two states, one word

We say we are “tired” when we are sleepy, and also when we are worn out. One word for two things. Yet they are two different states.

Being worn out is the weight left after effort: you want to stop, to sit down. Being sleepy is sleep on its way: heavy eyes, a nodding head, moments that slip. The first says you have worked. The second says you are about to fall asleep.

In 2005, in Bordeaux, twenty-two young men drove a thousand kilometres of motorway, once after a normal night and once after a short one. Before each stretch they gave two scores: how sleepy they felt, and how worn out. Philip et al. 2005

After the short night, the higher the sleepiness score, the more often they crossed the lane markings in the next stretch. The worn-out score was not linked to more crossings, after the short night or the normal one. Philip et al. 2005

The sleepiness you feel tells the truth

That is the good news. In a driving simulator at Loughborough, drivers who had slept little drove for two hours on a monotonous road. None of them fell asleep without first going through a long stretch of rising sleepiness. And those who fell asleep had felt it coming. Reyner & Horne 1998

A review of many studies reaches the same conclusion. When people say how sleepy they are on the Karolinska scale, from one to nine, they are as reliable as measurements taken with instruments. Above six, driving gets worse. Åkerstedt et al. 2014

It is not only true at twenty. In 2023, at Monash University in Australia, drivers aged 50 to 65 drove for two hours on a closed track after a night without sleep. They too felt sleepiness coming, and their Karolinska score signalled their driving problems. But compared with younger drivers, fewer of the questionnaire items signalled the problems. Cai et al. 2023

The mistake is in the reading

If the signal is reliable, why do people keep driving? In the Loughborough study, the serious lapses came when drivers were already fighting sleep. And some did not realise that such strong sleepiness makes falling asleep very likely. Reyner & Horne 1998

They felt it, and misread it. They treated sleepiness as a discomfort to push through. It is sleep announcing itself.

When the signal fades

There are two situations in which the feeling is no longer enough. Both are common.

After several short nights. In a laboratory at the University of Pennsylvania, adults who slept six hours or less for two weeks built up deficits worth up to two whole nights without sleep. Their sleepiness ratings rose in the first days, then barely moved. According to the authors, they did not notice the growing deficits. Van Dongen et al. 2003

When the situation covers it. Talking, moving about and daylight lower reported sleepiness by one or two points out of nine. Monotony raises it by as much. What you did not feel while chatting at the services can come back once the road turns straight and quiet. Åkerstedt et al. 2014

In those cases, watch how you do things

After several short nights, or just after something that kept you busy, the answer to “am I sleepy?” may come too quietly. Then you need a second check, on what you do rather than what you feel.

Do something simple you know by heart, and watch how you do it. Rushing where you are usually calm, lines read twice, losing the thread, small slips you don't usually make.

It is not a test, and it cannot tell you that you are fit to drive. But a change from your own normal tells you something, and it points the same way as the question. When in doubt, stop.