This structure sends signals to brain's pineal gland in the brain, telling it when to release a hormone called melatonin. Melatonin seems to signal the body when to fall to sleep and wake up. Kripke says the brightness level of light a person sees affects this timing system. If we spent most of the day outside like our ancestors, our body clocks would be set normally. However, most people are indoors much of the day.
Light brightness is measured in lux.
Outside on a bright, sunny day, the brightness level is about 10,000 lux. In the shade, the brightness goes down to 3,000 lux. Indoors, the level drops to 200 lux, and in front of the TV with the lights dimmed, the brightness is only 10 lux. Kripke says that purposeful exposure to bright light can help eliminate some sleeping problems. For example, if someone tends to wake up too early in the morning, his body clock may be running too early.