I have just read an article that suggests the larger the animal the less sleep it needs. The extremes given are that of a Hamster - that needs a huge amount of sleep and an Elephant - that needs next to nothing. Ok so that may not seem so daft when you think about the old heart rate and size theory (whereby the smaller the animal the faster the heart beat and hence the shorter life span etc.) because a smaller animal has clearly got to fit more sleep into their shorter life time. However if this applies to human beings then I think we should head towards the European Court of Justice and raise a case on grounds of equality! Just a thought!
As with all things there are exceptions (or incredible extremes) that break the rules: Dolphins are so clever that they can sleep whilst being awake... They can alternate which side of the brain they use to think whilst resting the other side in a form of sleep. It is notable that whilst they do this one eye closes. My father suggests he can do the same but doesn't have independent control of his eye lids!