Circadian Rhythm Exists
Statement
Humans have an internal \~24-hour clock governed by the
suprachiasmatic nucleus
Why it matters
Sleep timing isn't arbitrary --- there's a
biological window
Why Foundational
Foundational because the suprachiasmatic nucleus’s role as the master ~24-hour pacemaker is among the most cleanly characterised neurobiological systems — clock genes, retinohypothalamic input via melanopsin, and downstream peripheral entrainment have been replicated across decades and species. Sleep timing isn’t arbitrary; there’s a biological window.