In the lower Arctic, the light will still change subtly from day to night. "The sun it doesn't ever come up, but it gets closer to coming up," Williams says. But the farther north you travel, the more difficult it is to maintain circadian rhythms. Circadian rhythms control sleep patterns, digestion, body temperature, and other functions. In Arctic winter, many animals stop adhering to a 24-hour daily schedule altogether. Arctic ground squirrels keep their circadian rhythms through the constant daylight of Arctic summers, but not while hibernating in winter. Svalbard reindeer and rodents like lemmings adopt short cycles of feeding, digesting, and snoozing. "Instead of having this consolidated period of sleep each day, it's just kind of spread in these chunks throughout the day," Williams says. Other animals such as the chicken-like rock ptarmigan have no obvious rhythm to their sleep and activity patterns in winter.