A premature or low birth-weight baby needs to be fed more frequently than a full-term baby because he burns calories more quickly. You can understand why he needs to be fed so often if you think on a tiny hummingbird; it never stops feeding since its weight is so low compared to its volume that it needs constant food to stoke up the metabolic burners and keep the temperature normal. The smaller the baby, therefore, the more often he needs to feed and the less time he spends asleep. For premature babies the challenge of living outside the uterus is clearly an exhausting one. The lack of stimulation from being in an incubator and the inability to move very much means that, apart from frequent feeding, these premature babies spend most of their time sleeping.