Short and long photoperiod PRCs obtained in simulations. (A) 100 neurons were distributed according to two Gaussian distributions; one distribution was based on observations in the SCN from short day length, and the other from long day length . Each vertical line represents the peak time of a neuron. Many cells are active at CT 6, the middle of the day, and few cells are active at CT 18, the middle of the night. (B) The distributions in A were used to distribute 100 single unit PRCs. The y-axis represents differently phased single unit PRCs. The blue part of each line represents the delay part of the single unit PRC, the red part represents the advance part of the single unit PRC. The intensity of the color corresponds with the magnitude of the shift. The left figure shows the distribution for short days and the right figure shows the distribution for long days. (C) The resulting simulated population PRC for short and long days using single unit PRCs. The long day PRC shows lower amplitude than the short day PRC.