In the ancient Japanese timekeeping system, the length of one hour varied with the seasons (i.e. 24 hours was divided into 6 hours of daylight and 6 hours of night-time. Daylight hours were longer in the summertime and shorter in the winter). Laboratory mice were believed to be non-seasonal animals for a long time. Ono et al. reported that mice transmit seasonal information from the pituitary gland to the hypothalamus by using thyrotropin.