Optalert Drowsiness Mini-Series: Part Two

How your Body Regulates Drowsiness


Last episode, we uncovered the two different types of drowsiness and what they mean. This week we discuss how our body regulates drowsiness throughout the day by covering the sleep-wake cycle, a process that drives drowsiness in humans.

Our drowsiness level is regulated by the sleep-wake cycle that contains complex biological mechanisms. Simply put, there are several factors that influence where we sit on the drowsiness continuum.

  1. Adenosine is a chemical found in the brain that increases the desire to go to sleep.
  2. Sleep Drive represents the reservoir of adenosine that accumulates in the brain throughout the day and tips the sleep-wake cycle towards sleep.
  3. External Arousal Factors are factors that provide an increased level of wakefulness.
  4. Wake Drive represents the psycho-sensory drive where the arousal effect has its impact and tips the sleep-wake cycle towards wakefulness.

Collectively, these components interact continuously by inhibiting each other, with the balance between components encompassing a person’s sleep-wake cycle.

When we wake up in the morning, the sleep-wake “see-saw” is tipped towards the wake drive. Throughout the day, adenosine starts to accumulate, slowly tipping the scale toward the sleep drive. When the sleep drive outweighs the wake drive, we start to feel drowsy. We can undertake activities that act as external arousal factors. For example, standing up, watching an exciting sports event or turning up the radio when we are driving are arousal factors that would temporarily increase our wakefulness. However, a continuous build-up of adenosine would result in these activities only being effective for a short period of time. Sleep remedies this build-up by reducing adenosine levels in the brain.

Picture these two scenarios. One takes place in your car during your morning commute through the city centre, with speed limits, traffic lights and road safety cameras at every intersection. Your senses are filled to the brim as you manoeuvre through morning traffic, cyclists and jaywalkers. In this scenario, the sleep-wake “see-saw” is tipped towards the wake drive.  Adenosine levels in your brain are low, meaning the sleep drive “bucket” is empty, while firm pressure is placed on the wake drive, keeping you alert and awake.

Cut scene to a different scenario: a night-time drive in the countryside with nothing visible on either side. You are driving down a long stretch of road and the only things you see are the dotted lines on the road hypnotically passing by. The radio is off as everyone else in the car is fast asleep. In this scenario, the sleep-wake “see-saw” is tipped towards the sleep drive. Adenosine has been building up throughout the day and there are no external arousal factors to tip the scales back in favour of staying awake. As a result, you start to feel drowsy, or worse, microsleep while driving.

The good news is that there are ways (known as countermeasures) that can be deployed when we start to feel drowsy. Objectively knowing how drowsy someone is at a given time is critical in deploying the right countermeasures, as they can range from the very subtle (early-stage drowsiness) to the very severe (having fallen asleep). The earlier we implement these countermeasures, the more effective they are. In other words, we need an accurate measure of drowsiness even before the driver is aware they are starting to feel drowsy. A way to accurately and objectively quantify drowsiness levels is needed and is the topic of our next discussion.

For more details see Dr Murray John’s publication titled “Assessment of ‘Sleepiness’ in Human Drug Trials: A New Perspective”. You can download a copy here.’

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