The physiology of circadian entrainment involves the synchronization of internal biological clocks with environmental cues, primarily light. Mammalian circadian rhythms are controlled by the suprachiasmatic nuclei (SCN) in the hypothalamus, which acts as a master clock. Light-dark cycles serve as Zeitgeber (time givers) to entrain circadian rhythms. Recent advances in neurophysiology and molecular biology have enhanced understanding of circadian mechanisms, including retinal sensitivity to light through novel photopigments and circadian variations in retinal physiology. Retinohypothalamic communication, involving neurotransmitters like glutamate and aspartate, interacts with SCN receptors, leading to signal transduction pathways. Clock gene expression and its role in entrainment are also discussed, along with circadian disorders and retinal diseases related to entrainment deficits. Circadian entrainment is an adaptive feature, allowing organisms to adjust to environmental changes. Light pulses can induce phase shifts in circadian rhythms, with short pulses causing phasic entrainment and long pulses leading to parametric synchronization. The phase response curve (PRC) illustrates the effects of light on circadian phase, showing when light can induce phase delays or advances. Nonphotic entrainment, involving social stimuli or physical exercise, can also influence circadian rhythms, often in opposition to photic entrainment. Light is a critical Zeitgeber, with its effects mediated by the retinohypothalamic tract. The retina contains circadian rhythms, including melatonin synthesis and visual pigment gene expression, which help anticipate and adapt to light changes. Novel photoreceptors, such as those containing melanopsin, play a role in photic entrainment and masking. These photoreceptors, along with other pathways, contribute to circadian regulation, even in the absence of classical photoreceptors. The SCN and other brain regions interact with these photoreceptors to maintain circadian synchronization, with implications for circadian disorders and treatments. The study of circadian entrainment highlights the complexity of biological clocks and their importance in adapting to environmental cues.The physiology of circadian entrainment involves the synchronization of internal biological clocks with environmental cues, primarily light. Mammalian circadian rhythms are controlled by the suprachiasmatic nuclei (SCN) in the hypothalamus, which acts as a master clock. Light-dark cycles serve as Zeitgeber (time givers) to entrain circadian rhythms. Recent advances in neurophysiology and molecular biology have enhanced understanding of circadian mechanisms, including retinal sensitivity to light through novel photopigments and circadian variations in retinal physiology. Retinohypothalamic communication, involving neurotransmitters like glutamate and aspartate, interacts with SCN receptors, leading to signal transduction pathways. Clock gene expression and its role in entrainment are also discussed, along with circadian disorders and retinal diseases related to entrainment deficits. Circadian entrainment is an adaptive feature, allowing organisms to adjust to environmental changes. Light pulses can induce phase shifts in circadian rhythms, with short pulses causing phasic entrainment and long pulses leading to parametric synchronization. The phase response curve (PRC) illustrates the effects of light on circadian phase, showing when light can induce phase delays or advances. Nonphotic entrainment, involving social stimuli or physical exercise, can also influence circadian rhythms, often in opposition to photic entrainment. Light is a critical Zeitgeber, with its effects mediated by the retinohypothalamic tract. The retina contains circadian rhythms, including melatonin synthesis and visual pigment gene expression, which help anticipate and adapt to light changes. Novel photoreceptors, such as those containing melanopsin, play a role in photic entrainment and masking. These photoreceptors, along with other pathways, contribute to circadian regulation, even in the absence of classical photoreceptors. The SCN and other brain regions interact with these photoreceptors to maintain circadian synchronization, with implications for circadian disorders and treatments. The study of circadian entrainment highlights the complexity of biological clocks and their importance in adapting to environmental cues.