Table 2.

Sources and Primary Mechanisms of Circadian Rhythm Disruption

Source	Mechanism
	Environmental LD Cycle Disruption	Behavioral		Biological
		Feeding/Fasting Cycle Disruption	Rest/Activity Cycle Disruption	Genetic Disruption (eg, Clock Gene Mutations)	Physiological Disruption (eg, Retinal Dysfunction)
Work schedules (eg, shift work, social jetlag, early school start times)	✓	✓	✓	×	×
Jetlag	✓	✓	✓	×	×
Unusual photoperiods (eg, polar regions)	✓	×	×	×	×
Circadian rhythm sleep/wake disorders (eg, non-24-hour sleep/wake disorder)	✓	×	×	✓	✓
Senescence	×	×	×	×	✓
Disease states (eg, Alzheimer's, Smith-Magenis syndrome)	×	×	×	✓	✓