Secret to cheaper flights?

Last night Jacqueline was looking around online for flights to Seattle. She ended up falling asleep before she could book anything, but when she went to check again in the morning, she found that the same flights were about $30 cheaper.

Perhaps it was just a coincidence, but my theory is that people/agents put holds on flights throughout the day, and those holds are released at the end of the day (assumedly midnight, although I don't know which timezone), freeing up seats and thus reducing prices. Has anyone else seen this? Or is this common knowledge and I've just been clueless?