Ray…you are likely correct regarding lack of light and the negative affect on your balance. The vestibular system controlling balance is made up of 3 components…your inner ear mechanisms, your eyes and nerves in your feet. These all work in conjunction to keep you upright. When part of the system is damaged , the others try and compensate. In many cases , the eyes work overtime to try and offset the other deficits. As the light fades, the eyes can’t help as much…and the balance falters more. I think this is why it’s harder to stand with your feet together with eyes closed than when open.