This was a question I was asked the other day. My instinct ( a hangover from my beer brewing days) has always been to use dark brown bottles which are specifically there to keep the sunlight levels down, but many supermarket brands put their cider in clear glass.
Thinking it through, it’s the hops in beer that react with ultraviolet light. Since we have no hops in cider (although everyone likes to experiment now and again) we shouldn’t have too many problems.
Sunlight produces heat
However, during the brewing process we must remember that sunlight produces heat and too much heat is bad for cider makers for a number of reasons:
Firstly, if you are using old kit you may find that thermal expansion introduces gaps in seals (such as around your airlock or the lid of a brewing bucket) allowing nasties to get in and spoil the party.
Wild Yeast and heat
Those wild microbes and fungi do love a bit of heat as much as the pitched yeast, so if a batch of very old yeast appears to have worked in hot weather you may in fact be unwittingly brewing with wild yeast strains. Not a problem as such; until comparatively recently all cider was made this way.
Oxidation, when it occurs, happens a lot quicker in higher temperatures
Overheating may cause the yeast to die. That starts at 50 centigrade which is unlikely to be reached unless you leave the fermenting bucket or demijon in a greenhouse or car on a hot day.
Sun struck cider
Finally, if you leave finished cider bottles in full sun for a long time there is a thing called ‘sun struck’ where the brew takes on a slightly darker hue and a port or sherry like character. Apparently it is not in any way unpleasant, but is not what you would want. I have never experienced this myself, but as I said, I am a brown bottle man.