it's any time between midnight and 4 since that's the darkest points, at 4 the sun starts to rise in certain places. i've dealt with spirits, and i've noticed more activity around 3 am, though this is only around certain times of the year.
it was coined, as Lancer mentioned, by Shakespeare, but Washington Irving also used it in The Legend of Sleepy Hollow to warn when they Horseman would be seen. [credit would go to Shakespeare though since Irving came about 200 years later]
whoever coined it though, it has been noted for centuries that around this time of night spirits seem to grow stronger, along with all types of magical powers. if you feel this is the best time to cast, go for it. personally, unless i have to stay awake all night, i can't make it past 10:30. [unless i'm watching tv lol]