cycle overall is surprisingly similar to other mammals. This shows that despite differences in sleeping patterns, the sleep wake mechanism is conserved across species. The anatomy of the cetacean brain varies slightly from that of other mammals, with the area governing sleep-wake cycles (caudal to the posterior commissures) being significantly larger. It is difficult to use traditional methods to determine whether a porpoise is sleeping, since half of the brain is awake at any given time. Parabolic dives have been shown to potentially be correlated to sleeping periods in porpoises, with decreased bioacoustics being transmitted during these periods. This means that during parabolic dives, porpoises generally do not employ echolocation clicks (about fifty percent of the time). This may be related to the side of the brain that is asleep. For instance, the left hemisphere signals to the right side of the brain, which controls the production of echolocation clicks. Thus, when the left side of the brain is asleep, echolocation cannot be produced. Along with using less bioacoustics, porpoises roll less, use a lower vertical descent rate, and are overall less active while performing parabolic dives. Parabolic dives have shown to be of shallow depth and use a low amount of energy. These are different behaviors than the those displayed during foraging dives. Moreover, these behaviors are consistent with stereotypical sleeping behavior. Interestingly, parabolic dives are more common during the daytime than during the nighttime and only take up a small amount of a wild porpoise's total time, as opposed to the time spent sleeping in other mammals. Meanwhile, captive cetaceans have been shown to spend up to fifty percent of their time sleeping and up to sixty-six percent of their time resting. A thought that deserves further consideration is that wild porpoises spend a significant fraction of their time near the surface of the water, and it is difficult to determine if this time is being spent resting or sleeping. Behaviour Life cycle Porpoises are fully aquatic creatures. Females deliver a single calf after a gestation period lasting about a year. Calving takes place entirely under water, with the foetus positioned for tail-first delivery to help prevent drowning. Females have mammary glands, but the shape of a newborn calf's mouth does not allow it to obtain a seal around the nipple— instead of the calf sucking milk, the mother squirts milk into the calf's mouth. This milk contains