Measuring drowsiness in clinical trials

Using Optalert’s Johns Drowsiness Scale (JDS), various studies and trials have accurately measured a user’s state of alertness and drowsiness.

In the study below, first published in 2008, we understand the effect of caffeine on already-alert people using the precision and sensitivity of Optalert’s patented JDS technology.

By identifying a direct link to human alertness, we gain a quantifiable measure of wakefulness, literally putting a number on how drowsy or alert you are (not just how alert or tired you may feel).

Michael, N., Johns, M., Owen, C, Patterson, J., (2008). Effects of caffeine on alertness as measured by infrared reflectance oculography. Psychopharmacology, 200: 255-260.





What effect does caffeine have when you are already alert?

Caffeine is a well-known and widely-used stimulant capable of increasing a person’s alertness.

It’s estimated that up to 90% of North Americans consume caffeine in either coffee, tea or cola drinks on a daily basis.

We already know that a cup of coffee provides the little pick-me- up you need when you’ve had a poor night’s sleep or are working on a boring and or repetitive task.

There are some important questions caffeine consumption raises, including:

  • What effect does caffeine have on someone who is already alert?
  • Is being alert a single discrete state or is it one that has many different ‘shades’ or degrees of alertness?
  • Can ocular variables be used to detect the various degrees of alertness?

Optalert can put an objective and accurate measure on alertness through its Johns Drowsiness Scale (JDS), named after its inventor, Optalert Founding Director Dr Murray Johns.

The scale ranges from 0 to 10 where 0 = very alert and 10 = very drowsy.

Its original intent was to objectively identify the earliest signs of drowsiness in a user, a state which can be particularly dangerous if the user is performing a task that requires a high degree of vigilance, like driving.

Through the study Effects of caffeine on alertness as measured by infrared reflectance oculography, we can also understand how the JDS can precisely provide a quantifiable and incremental change on alertness.

The effects of caffeine on alertness

Swinburne University researcher, Dr Natalie Michael and her colleagues used a well-accepted and scientifically-rigorous experimental design to investigate the effects of caffeine on alertness.

They conducted a study based on the following conditions:
Caffeine tablet and placebo tablet are not known to the investigators or participants.
The same participants ingest the caffeine tablets and placebo tablets on different days.
Half the participants take caffeine (Drug A) on their first experimental day and the other half take the placebo (Drug B).

Researchers recorded the participants’ performance on a 10-minute vigilance test (a simple reaction time task) scheduled at baseline and then 30, 60, 120, 180, and 240 minutes after ingestion.

Participants also self-reported their alertness using the Karolinska Sleepiness Scale (KSS).

Each participant’s ocular movements were recorded using special glasses containing sensors (called infrared reflectance oculography), yielding ocular variables that were combined to form an objective measure of alertness (JDS).

Most importantly, participants attended the experimental days being well rested after a good night’s sleep.


At baseline recording, there was no difference between caffeine and placebo groups in:

  • Performance (reaction time)
  • Subjective assessment of alertness (KSS)
  • Objective assessment of alertness (JDS scores).

However, after ingestion, the caffeine group:

  • Performed significantly faster
  • Reported being more alert
  • Objectively were more alert

JDS’ sensitive measurement

The most impressive aspect of the results using JDS scores were that they mimicked the profile of caffeine absorption: alertness increased rapidly after only 30 minutes (peak absorption) before beginning to subside after 3-4 hours (consistent with caffeine half-life).

The same cannot be said for subjective reporting which showed a significant difference after 30 minutes, but could not be differentiated from the placebo group at each recording session thereafter.


Historically, caffeine has been shown to have little effect on ocular variables such as blink rate and saccadic eye movements (fast eye movements made when looking around or reading a book).

This study showed for the first time ever that a combination of ocular variables (as used to calculate the JDS score) were sensitive enough to detect small fluctuations of alertness in already alert people.

Conversely, participants’ own awareness of small changes in alertness using a self-reporting scale appeared to lack the fidelity to distinguish the same differences.

Finally, JDS scores reflected the known peak absorption and half-life of a drug (caffeine), which suggests the scale can be used to help either identify or confirm the pharmacokinetic properties of a drug.

See more about how the JDS is now being used in clinical trials for the pharmaceutical industry.