Supposed 隔夜菜 means something kept "overnight", if night
begins at 10pm and ends at 6am, that's 8 hours?
How about 12pm - 8pm, they are not 隔夜菜, but
was kept the same amount of time, perhaps under worst
conditions for afternoon is warmer?

Hate this kind of overly generalized, no specific
type of statements...