The human eye is sensitive enough to detect a single photon, and can reliably detect 30. Interestingly, the levels of light observed (up to 3,000 photons per second) are within the range of human visibility.

That said, it takes more than 30 minutes for our eyes to acclimate to complete darkness, so that's probably why we don't normally observe this phenomenon. (Plus the fact we're rarely ever in complete darkness.)

Bonus facts about the human eye:

• We can see infrared
• We can distinguish the polarity of light (and the vikings may have used it to navigate their ships).
• The eye is one of the fastest healing parts of the human body
• Our eyes don't change much in size from infancy to adulthood (hence why babies have relatively large eyes)
• The most common form of color blindness relates to the green chromoceptor being tuned to a different wavelength. It's on the X-chromosome so 8% of men have "color blindness". Evolutionarily, groups of men can see through camouflage better (good for hunting), while some women have 4 distinct chromoceptors (R-G1-G2-B) and can distinguish more shades of green (good for gathering).
• The fovea is responsible for detailed and color vision. It covers 1% of our visual field, but 50% of the optic nerve and visual cortex. The upper limit of human eye resolution is about 1,000 ppi at one foot. The cone density is so high that there aren't many rods, so our night vision is better peripherally than if we look right at something.
• The human eye can see over 1,000 FPS (although the whole concept of FPS doesn't apply, since it continuously transmits the changes, not full images). Movies are shown at 23.976 FPS out of tradition. Soap operas were shot with superior technology, but crappy plots, so most people have a poor perception of 60 FPS video & it's never really caught on. Rumor is that the Avatar sequel will have a higher framerate since panning landscape shots require it.