ok, it's not actually sleeping:
"Garter snakes probably shed their skin once or twice a year in Manitoba. You can easily tell when the process is imminent. The snake's colour begins to dull and its eye will grow cloudy. About a week before the skin is shed the eye will be at its milkiest in appearance. You may notice that the snake seems less active at this time and may refuse to eat. With its vision impaired the snake would be vulnerable in the wild, and would probably hole-up somewhere at the height of this change. The eye cloudiness will disappear over several days, as the old skin thins and prepares to be cast off."
