The homeostatic sleep drive increases with sleep debt—the more hours awake, the higher the drive to sleep. Although processes exist that cause continued alertness within an individual with excessive sleep debt, the only way to satisfy the homeostatic drive to sleep is by sleeping. High cognitive workload promotes sleep homeostatic responses by increasing subjective sleepiness and fatigue, but it also delays sleep onset, producing a global sleep homeostatic response by reducing wake after sleep onset (2).

With its alerting effects, the circadian rhythm opposes the homeostatic drive to sleep. Through specialized retinal photoreceptors containing melanopsin, the suprachiasmatic nucleus (SCN), located in the anterior hypothalamus, receives input from light. The input of light to the SCN inhibits melatonin secretion by the pineal gland
(1), producing alertness. When the SCN receives input from low levels of light (dark), melatonin is secreted from the pineal gland, causing a decrease in alertness.

The pineal gland produces melatonin from the amino acid tryptophan. Melatonin is secreted not only into the blood but also into the cerebrospinal fluid. Daytime plasma melatonin concentrations are at least 10-fold lower than nocturnal concentrations (3, 4). The secretion of melatonin begins at 3 or 4 months of age, concurrent with sleep consolidation at night. Nighttime melatonin levels then increase rapidly, peaking at 1 to 3 years of age, at which point it begins to decline (5): melatonin nocturnal serum concentrations decline across puberty (6). Peak nocturnal melatonin concentrations in 70-year-olds decrease to only a quarter or less of what they are in young adults (7).

Ocular light exposure induces a range of neurobehavioral, neuroendocrine, and circadian responses, including melatonin suppression, circadian phase resetting, and enhancement of alertness and performance. These responses are most sensitive to blue (short-wavelength, 450 to 480 nm) visible light (8). Before bedtime, sleep improves when subjects looked through amber lenses (blocking blue wavelength light) instead of clear lenses (9). Nighttime light exposure acutely suppresses melatonin and increases alertness in a dose-dependent manner. Independent of melatonin suppression, daytime white light exposure has also been shown to increase alertness (8).

Although inherent and set genetically, the circadian system is modifiable. Zeitgebers, meaning “time givers” in German, entrain or align the internal clock, which cycles with a period slightly longer than 24 hours: about 24.2 hours in adults (10) and 24.3 hours in adolescents (11). Although zeitgebers include meals, exercise, and social contact, the most potent stimuli to entrain the circadian phase is bright light (12).