A Lack of Sunlight

UV is needed to utilize oxygen in the blood. UV increases nitric oxide and the relaxation of blood vessels. UV increase free endothelial NOS/eNOS (in vessels/capillaries) and total neuronal NOS/nNOS (in neurons) and increases nitric oxide through other means (R), which allows oxygen to diffuse into tissues better. UV also relaxes your blood vessels by mechanisms that might be independent of NO (R).