Biological clock refers to an innate process which facilitates sleeping, aging, fertility, and other physiological activities. This internal mechanism may change weekly, monthly, or other regular cycles. Biological clock is most often associated with the sleep-wake cycle or circadian rhythm. For instance, we feel sleepy at night because darkness triggers the production of melatonin hormones which cause drowsiness. On the other hand, brightness suppresses the release of melatonin and that is why we feel more alert during day time. This schedule is also strengthened by habit. For example, you usually go to bed at 10:00 PM and wake up at 6:00 AM; hence, you tend to feel sleepy by 10 PM even on a Saturday night and you feel your body waking up at 6:00 AM even on a weekend.