I set my thermostat back to 64F at night and my plants do just fine. Most of them prefer it a bit cooler when darkness sets in. I grew up with a greenhouse right off our dining room that was set to 45F in the colder months. The sun (if there was any in the New England winter months!) warmed it during the day. In hindsight, it might have been too cold for them but I don't remember any plants dying because of this. Most houseplants are tropical in origin and do like it warm especially during the daytime.