Some 320 million years ago, amniotes began to invade our planet's terrestrial niches. These amniote ancestors gave rise to two lineages that have led to today's mammals (synapsids) and sauropsids (reptiles and birds). Mammals and non-avian reptiles are the only vertebrates with a layered cerebral cortex. To which degree their circadian systems share common mechanisms or have unique adaptions in regulating sleep timing and architecture is unknown. Early circadian studies in reptiles lack conclusive interpretations and technologies have since greatly improved. The bearded dragon is an established model organism for electrophysiology and shows alternating slow-wave and rapid-eye-movement sleep states (Shein-Idelson, Ondracek et al. 2016). Its superficially placed pineal complex includes a third eye, providing a unique input pathway for circadian entrainment. We show that the pineal gland, an autonomous circadian oscillator, rhythmically secretes melatonin even when isolated in culture. Moreover, lizards whose pineal glands have been surgically removed (pinealectomized) are unable to entrain their sleep onset and offset timings to altered light/dark (16:8) cycles and become arrhythmic in constant conditions (DD). The bearded dragon thus presents a distinct opportunity to study the interaction between a central circadian oscillator (the pineal gland) and a behaviour (sleep) coupled through a molecular agent (melatonin). Not only do we hope to highlight principles and rules that determine interactions among component parts of the vertebrate circadian timing system but also to unravel the ancestral role of circadian regulated sleep (containing SWS and REMS) through non-avian reptiles who branched away from synapsids early in amniote evolution.