Dr Daan van der Veen is a lecturer in sleep and chronobiology at Surrey University. He has worked with voles (Microtus arvalis) because these animals naturally live in a fast, ultradian rhythm. They feed and socialise for a few hours and then sleep for an hour or so before repeating this cycle three or four times a day.The animals are kept in deep litter so they can burrow,and provided with an exercise wheel in each cage to allow them to use up their restless energy. In chronobiology, an ultradian rhythm is a recurrent period or cycle repeated throughout a 24-hour day. In contrast, circadian rhythms complete one cycle daily, while infradian rhythms such as the human menstrual cycle have periods longer than a day. The Oxford English Dictionary's definition of Ultradian specifies that it refers to cycles with a period shorter than a day but longer than an hour.The descriptive term ultradian is used in sleep research in reference to the 90–120 minute cycling of the sleep stages during human sleep.Some references to learn more:http://www.pnas.org/content/103/9/3393.abstracthttp://www.tandfonline.com/doi/full/10.3109/07420528.2011.591953http://www.fasebj.org/content/31/2/743.longAnd more generally:https://en.wikipedia.org/wiki/Ultradian_rhythm