Endocrine Secretory Patterns Can Be Influenced by Factors Such as Sleep or Light and Can Produce Circadian Rhythms

Endocrine secretion patterns can occur outside the con­trol of negative-feedback inhibition. Hormone patterns can change on an approximate 24-hour basis, a process referred to as a diurnal rhythm, or circadian rhythm. Circadian is the preferred term because diurnal refers to activity in the daytime; nocturnal should be used for the rhythms that are active at night. Most of the daily rhythms have some aspects of light, or lack of light, as a major influence in the rhythm. Rhythmic changes in hormone patterns that occur at shorter intervals, often in the range of an hour, are called ultradian rhythms.