The document discusses the biological clock, an internal system that controls circadian rhythms in organisms. It describes the biological clock's location in the brain near where the optic nerves cross. It then outlines different types of biological rhythms like circadian, ultradian, and infradian rhythms. The presentation concludes that biological clocks play a vital role in regulating bodily processes like sleep, temperature, and reproduction and ensuring sensitivity to substances depends on the time of day.