The onion (Allium cepa) is very daylight sensitive. So much so that there are varieties developed especially for the North that require a long photoperiod (14 hours or more of sun per day in the summer) to produce a bulb; in the South, they only produce roots. Most onions offered for gardens in Europe or North America are long-day varieties.
But there are also intermediate-day onions that will form bulbs under days only 12 to 13 hours long, ideal for Southern Europe, North Africa, the Southern US and Central America.
And there are even short-day onions designed for sowing over the winter in mild climates: they only need only 11 to 12 hour days to form bulbs.
Normally Not a Problem, But…
Onion day length needn’t be a concern if you buy your onion seed or onion sets locally. Local merchants sell only varieties adapted to the region. However, if you’re given to buying seeds by mail from exotic locals, be careful. If you buy seeds form a catalog that spans both North and South, it is important to read the description to know for which region the onion is intended.