Active children downstairs during the daytime are MORE likely to have accidents - need adequate supervision. Alert, adult presence is required - can be in the room or elsewhere for short periods, but still alert to hear issues arising so can respond if necessary.
Night-time means less activity from children who are more confined than in daytime,as in bedrooms, many hours spent asleep - means LESS likely to have accidents - time for adults to sleep.
The issue isn't whether there is risk - there is ALWAYS some degree of risk and this can never be totally avoided. The issue is to MEASURE the risk and put in place measures to minimise it. In the nighttime, it is fine for adults to sleep, as the risks are LOW even if never totally removed.
In the daytime the risk of very small children downstairs alone without a conscious, alert presence is HIGH,which is why they shouldn't be left. The way to minimise the risk is to be awake and either in the room with them or nearby so you can anything untoward and come quickly. Being asleep upstairs for an hour whilst hungover means there is no adult who is alert and who can respond quickly to a problem = inadequate supervision.
Of course children quickly become more sensible. One child may be less risky if left alone than two or three. Older children can certainly be given more leeway to be alone for longer.
For those who think what the OP did was fine......what is the YOUNGEST ages for 2 children that you would find her napping for an hour upstairs to be acceptable/reasonable? Would it be okay if there was a small baby? 9 month old crawler? 2 year old?
Everyone does have to make a judgement about these things but for me a 3 year old is too young to be responsible for themselves for a whole hour with no adult listening out and only a 5 year old to take responsibility.