For some performance variables, and some individuals, there is a dip in performance during the midafternoon hours (referred to as the post-lunch dip) that is linked to an increase in sleep propensity at that time of day. The post-lunch dip is a real phenomenon that can occur even when the individual has had no lunch and is unaware of the time of day. This dip has its roots in human biology, and may be linked to the size of the 12-hour harmonic in the circadian system. It is certainly exacerbated by a high-carbohydrate lunch, and may be more likely to occur in extreme morning-type individuals.