It's best to keep your drinks in a cool, dark place. Prolonged exposure to sunlight has a detrimental effect on liquor, and the longer liquor is exposed to the sun, the worse the effects will be. The exact result of light exposure depends to a degree on what type of liquor is in the bottle left in the sun.
If you're a regular beer drinker, you have likely had the unpleasant experience of opening a bottle of "skunky" beer. This term refers to the rancid smell and flavor that sometimes affects a bottle of beer that has gone bad. Beer that tastes off due to light is called skunky because the reaction between ultraviolet light and the chemicals produced by boiled hops produces a chemical similar to some of those that skunks produce when spraying.
There is a reason wine cellars are traditionally built with no windows in them. Wine bottles allow the light in. If left out in the sun, the ultraviolet rays eventually destroy the wine within the bottle. Wine should always be stored in a dark cabinet or cellar, or at the least, out of any direct sunlight to ensure a problem-free aging process.
Whiskey is like other liquors and will be damaged by the sun. Like wine, a good whiskey gets better with age, so it is best to store it in a way that preserves it as well as possible. Whiskey does not get damaged as easily as beer or wine, but over time the flavor could still be adversely affected by enough sun. Certainly, this is not something you want to have happen to a 20-year-old scotch.
If a liquor bottle can't be stored in a dark cellar at all times, the best protection it has against the effects of the sun is the bottle it is stored in. Clear glass does not provide protection against the sun. Brown glass filters out more of the ultraviolet light that damages the bottle's contents. This is why beer is typically bottled in brown bottles. Red wines and scotch are often bottled in green bottles, which do offer slightly more protection than clear bottles.