Many organisms have a biological calendar that they can use to time seasonal events. Humans are not thought of as seasonal animals, in the sense that many other animals are seasonal breeders and their lives are tightly governed by photoperiod. Daylength is affected by latitude. For example, at 60 degrees north latitude, the longest day of the year is about 18 hours and the shortest day of the year is about 6 hours. At 30 degrees, the shortest daylength is 10 hours and the longest daylength is 14 hours. A response of an organism to the changes in day and night length is a photoperiodic response. Most measured critical photoperiods fall between 10 hours and 14 hours of light. The critical photoperiod within a species varies systematically with latitude. A photoperiodic behavior that has intrigued biologists is animal migration. Animal migration depends upon the season.