What condition causes nails to appear blue?

Nails appear blue due to a condition known as cyanosis, which is primarily caused by poor blood circulation or a lack of oxygen in the bloodstream. This condition can result from various factors, including respiratory issues, cardiovascular problems, or exposure to cold temperatures that restrict blood flow. When there is insufficient oxygen in the blood, the hemoglobin in red blood cells takes on a darker color, which can give the nails and sometimes the skin, a bluish tint.

While dehydration, overheating, and sleepiness can affect overall health, they do not specifically cause the nails to change color to blue. Dehydration may lead to dryness and brittleness of the nails, overheating can cause sweating, and sleepiness affects general energy levels but does not directly influence the oxygen content in the blood or circulation in a way that would lead to cyanosis. Therefore, the correct answer highlights a fundamental understanding of how oxygen levels and blood circulation directly affect nail coloration.