This rhythm is endogenous, meaning it is maintained even in the absence of environmental cues. Human beings placed environments with no light, no time cues, no social cues, etc., will still display a 24-hour rhythm. However, external timing cues do modulate and adapt the rhythm to the environment. For example, sunlight, in humans, is a powerful cue to have the waking phase of our 24-hour cycle during the day. Other mammals, like rats and mice, have most of their waking phase at night. These rhythms were created through many years of evolutionary pressure. Light cues, which modulate our circadian rhythm, are extremely powerful. Circadian rhythm is endogenous so an internal pacemaker is necessary. The suprachiasmatic nucleus in the hypothalamus is a major part of the internal pacemaker of humans. This part of the brain is connected to the retina via the retinohypothalamic tract.2 This direct and powerful connection shows just how important light is for controlling circadian rhythm.
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