Yes, hummingbirds do indeed have a very high metabolic rate, but nevertheless hummingbirds do not eat until they are full at every opportunity. During daylight hours, the hummingbird will eat frequently for short periods of time. A hummingbird will feed 5-8 per hour with each one only lasting about 30 seconds in length. At night, the hummingbird will eat as heavily as the nectar supply will allow. The hummingbird may store a third of its weight in nectar to ensure that it will survive its overnight fast while maintaining its normal resting metabolic rate.
It is important to keep in mind that a hummingbird can starve to death in as little as two hours. This is why it must store so much of its weight in nectar to ensure it will survive the night.