Physiological measurements (of alertness and sleep)

The use of physiological measurements is one of many methods used to study alertness and sleep.

Numerous physiological methods can be utilised to measure levels of alertness in normal waking consciousness, and the stages of sleep. It is easy to confuse electroencephalographs (EEGs), electromyographs (EMGs) and electro-oculargraphs (EOGs). As a rule, remember that an EEG measures electrical activity in the brain, an EMG measures electrical activity in the muscles (‘M’ is for muscles), and an EOG measures electrical activity in the muscles around the eyes (‘O’ looks like an eye).

In defining EEG, EMG, EOG, it is important to use the acronym DARE:

  • Detects
  • Amplifies and
  • Records
  • Electrical activity

This is consistent for all three measures.

Electroencephalograph (EEG)

EEGs detect, amplify and record electrical activity in the brain (in the form of brainwaves). For example:

In extreme cases, brainwave measurements can be used to assess the mental state of an individual. For example, psychopaths often have theta waves, and those with a brain tumour may show delta waves.

Electromyograph (EMG)

EMGs detect, amplify and record electrical activity in the muscles. For example:

Electro-oculargraph (EOG)

EOGs detect, amplify and record electrical activity in the muscles around the eyes (which allow them to move). For example:

  • When awake and alert (in normal waking consciousness), the eyes may or may not move rapidly, depending on one’s activity at the time
  • When in NREM sleep, there will be very little rapid eye movement
  • When in REM sleep, there will be bursts of rapid eye movement

EEG, EMG and EOG tests all use electrodes placed on the skin to record electrical activity.

Heart rate

Heart rate is another physiological measurement which can be used to study levels alertness and stages of sleep. During normal waking consciousness, our heart rate will vary depending on our activity at the time. That is, we are more likely to have a low heart rate when resting when compared to undertaking strenuous exercise.

Heart rate during NREM sleep is likely to be stable, whereas during REM sleep, it can vary significantly, as per normal waking consciousness.

Body temperature

Body temperature tends to be regulated during NREM sleep, whereas it is not regulated in REM sleep. This means that during REM, our body temperature is more likely to depend on the surrounding environment.

Galvanic skin response (GSR)

Galvanic skin response refers to the electrical conductivity of the skin. The skin’s electrical conductivity will increase as it becomes more moist. As there tends to be a direct correlation between state of consciousness and skin conductivity, the GSR can be used to analyse alertness and sleep. For example:

  • Normal waking consciousness: GSR depends on activity. For example, it can be heightened through exercise or arousal, and can be lowered through relaxation.
  • Altered state of consciousness: GSR depends on type of ASC. For example, it can be heightened through drugs, and can be lowered through meditation.

Benefits and downfalls of physiological measurements


  • Data is objective
  • Data is consistent and easily compared
  • Data can be measured automatically


  • Does not record subjective data
  • Does not take into account personality
  • Does not account for extraneous variables