A transcranial magnetic stimulation study on the influence of chronotype and the circadian rhythm on motor evokedpotentials and motor threshold

Transcranial magnetic stimulation is used to non-invasively stimulate parts of the brain. A stimulus, directed at the motor cortex, can evoke a reaction in the targeted muscle (motor evoked potential, MEP). This only occurs, when the stimulus is strong enough to reach a certain threshold (motor threshold). The motor evoked potential and motor threshold are important measures of cortical excitability. Cortical excitability describes how easily neurons are activated. Neurons, especially in the deeper layers of the brain, are influenced by hormones and neurotransmitters, resulting in inhibitory and excitatory signals. Many hormones and neurotransmitters, such as GABA-A and cortisol show a circadian rhythm, meaning a variation over the day, under the influence of the human ‘biological clock’. The timing of this biological clock differs between persons, depending on one’s chronotype (being a morning type or evening type person). Some parameters of cortical excitability show this same circadian rhythm and show differences in peak time values between chronotypes. We studied the motor threshold and amplitude of the motor evoked potential in 16 subjects (six evening type, six neutral and four morning type subjects). We saw a significant variation over the day in the motor evoked potentials from the morning type group, with a decrease in amplitude during the day. Research on cortical excitability (for instance in the field of epilepsy) should account for chronotype and time of day as possible confounders in their results.