are homologous to those found in the fruit fly. these cells contain the photopigment melanopsin and their signals follow a pathway called the retinohypothalamic tract, leading to the scn. if cells from the scn are removed and cultured, they maintain their own rhythm in the absence of external cues. the scn takes the information on the lengths of the day and night from the retina, interprets it, and passes it on to the pineal gland, a tiny structure shaped like a pine cone and located on the epithalamus. in response, the pineal secretes the hormone melatonin. secretion of melatonin peaks at night and ebbs during the day and its presence provides information about night-length. several studies have indicated that pineal melatonin feeds back on scn rhythmicity to modulate circadian patterns of activity and other processes. however, the nature and system-level significance of this feedback are unknown. the circadian rhythms of humans can be entrained to slightly shorter and longer periods than the earth's 24 hours. researchers at harvard have shown that human subjects can at least be entrained to a 23.5-hour cycle and a 24.65-hour cycle. humans when eyes receive light from the sun, the pineal gland's production of melatonin is inhibited, and the hormones produced keep the human awake. when the eyes do not receive light, melatonin is produced in the pineal gland and the human becomes tired.