In their research article -- A highly tunable dopaminergic oscillator generates ultradian rhythms of behavioral arousal-- Blum et al. at McGill University and the Douglas Mental Health University Institute, Canada identify a second internal clock within the brain, which they name ‘the DUO’. They show that this clock normally works in concert with the circadian clock to regulate daily patterns of activity and alertness.
Experiments in mice revealed that the DUO uses the brain chemical dopamine to generate bursts of activity roughly every four hours. Moreover, it continues to work when the circadian clock has been destroyed. Measurements of dopamine in freely moving mice showed that levels of the chemical fluctuate in synchrony with the animals' activity levels. Moreover, drugs that flood the brain with dopamine, such as methamphetamine, disrupt the 4-hour cycle by lengthening the period between bursts of activity, whereas drugs that block dopamine receptors have the opposite effect.
Examples of the media coverage recieved by this research can be found below;
