[vladms]

I think in real life there are more constraints. For example there are people that sleep better at a lower temperature than the daily one (so leaving the thermostat at the same temperature it's a minus for them).

Regarding "what is better" from energy efficiency, I would prefer a system that "check it" because my guess is that it depends a lot based on the individual situation. I mean everybody is going crazy over "IA" but a couple of sensors and a system smart enough to adjust your usage based on your particular situation and preferences (like "eco", etc.) is an exception.

[alistairSH]

In slightly cooler climates, the answer for sleep is to open windows. This works in much of Europe, even through summer.

But of course, not really feasible in Atlanta or Phoenix. Nighttime temps are too warm.

[eru]

We are talking about places that need heating here.

In general, you should either run the heater or have your window open. Both at the same time is bad news for your energy bill.

[deno]

Open windows in a city often means you are invinting in pollution, both air and noise.

[JoshTriplett]

Exactly. Current HVAC systems have extensive filtration; some HVAC systems have HEPA filters.

[dolmen]

Opening windows also helps to evacuate inner pollution coming from furnitures, wall paint, parquet and other chemicals...