Brain quality index (BQI) is the quality of the brain about how well the brain manipulates and the level of capacities. Brain quality index is the resistance of making mistakes, forgetting things, and doing things by accident. BQI always change amongst everybody, including predictable variations throughout the day and year. Generally BQI is affected by amounts and lengths of daylight throughout the year, because the amounts of natural lighting is good for their brains, lot better than artificial lighting. BQI is lowest at around 3am and highest at around 3pm. Throughout the year, BQI is lowest in December and highest in June. During the night, decreased BQI caused by the lack of natural lighting can cause more violence, along with increased frequencies of making mistakes, forgetting things, and doing things by accident. Increased BQI also makes people to think faster and clearer. Owing to their BQI, most people prefer to work during the day and read or play at night.