What is drowsiness and how can it be measured?

When we commonly speak of drowsiness, we may use a range of terms interchangeably, such as fatigue, tiredness, or sleepiness. In common speech, this is fine. But for the purposes of understanding the underlying science, we need to be more precise around what each of these words mean.

Drowsiness is not fatigue

Fatigue is defined as a reduction in the efficiency of a muscle or organ after prolonged activity. It occurs after strenuous physical exercise or labour. If you lie on the couch after intense exercise, you regain energy.

Drowsiness emerges in the continuous state between being asleep and awake. And when drowsiness builds up, rest leads to sleep. If a drowsy person lies on the couch, they will nod off.

Drowsiness is physiological in nature

It is important to understand that drowsiness is not a feeling. It is a neurological state with a physiological process behind it.

There are two competing drives that determine if you are drowsy: the sleep drive and the wake drive. We can picture them like a seesaw. It shifts one way or the other as we spend time asleep or awake. If the sleep drive outweighs the wake drive, you get drowsy and eventually fall asleep.

Here is a quick overview of the mechanisms at play.

Sleep drive

Time awake

Throughout the day adenosine builds up in the brain, slowly dripping into our sleep bucket. It binds to neurons and slows down their activity. When we go to sleep we take our bucket of adenosine and tip it out.

Prior sleep

If you have poor sleep it can accumulate between days, commonly referred to as your sleep bank. Some people refer to this as a “sleep debt”. The adenosine can accumulate between days with poor sleep hygiene.

Time of day

There is another effect and that is time of day. There is a melatonin tap that drips into your sleep bucket as your core body temperature drops, particularly between 2:00 and 4:00 a.m.

Wake drive

What stops you getting drowsy is your wake drive. This is driven by arousal: visual stimuli, smell, taste, and also mental stimulation.

These are all important in understanding drowsiness in driving scenarios.

You are unlikely to have a drowsiness-related accident when driving through a new city, or in a built-up area with a lot of decisions.

Take the same person and put them onto a long, boring stretch of road and you have an increased risk.

Drowsiness is expressed biologically in two ways

There are two aspects of drowsiness that can be measured:

How can we measure tiredness and impairment?

The most common measurement of subjective tiredness used in driver monitoring systems is the Karolinska Sleepiness Scale (KSS). This involves asking a subject how tired they are on a scale of 1 to 9 (see table).

The main issues with the KSS are:

  • As a subject becomes more impaired, their ability to assess tiredness also becomes impaired.
  • It is not stable over time – my “7” last month may not be the same as my “7” this month.
  • It varies between subjects – your “7” may differ from my “7”.
  • It is extremely low resolution – only two values, “7” and “8”, are of interest to developers of driver monitoring systems. This is not a fine enough resolution to detect drowsiness in advance of a high-risk scenario.

Measuring impairment is the more useful approach for predicting a performance failure in a driver. Optalert’s Johns Drowsiness Scale (JDS) is a measure of impairment that Harvard Medical School has deemed commensurate with gold standard laboratory measures. It involves tracking eyelid movements (what is called “blepharometry”) and calculating a composite measure involving 64 characteristics of this signal. This is the best predictor of a performance failure and is therefore the best measure of impairment.

In the lab, we measure performance failures with the Johns Test of Vigilance (JTV), which evaluates the rate at which a person responds to a visual stimulus within two seconds. As a subject becomes more impaired from drowsiness, we can precisely measure the degradation in their ability to respond to visual stimuli. On the road, the most common impairment metric is wheels out of lane.

The JDS is the most accurate predictor of these performance failures both in the lab and on the road.

  1. Extremely alert
  2. Very alert
  3. Alert
  4. Fairly alert
  5. Neither alert nor sleepy
  6. Some signs of sleepiness
  7. Sleepy, but no effort to keep alert
  8. Sleepy, but some effort to keep alert
  9. Very sleepy, great effort to keep alert, fighting sleep
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How can Optalert help you with measuring drowsiness?

If you’re in the mining, transport, or fleet industries, visit our Mining & Transport page to get an overview of our product suite and how it can help you reduce risk, increase productivity, and manage fatigue and drowsiness across a fleet of drivers.

If you’re in the automotive industry, visit our Automotive page and learn about how you can integrate our drowsiness measurement technology via a software development kit (SDK) into your driver monitoring system (DMS).