The OP is in Iowa, the whole state experiences a ''real winter'' USDA zone 6, 5 or 4 depending on location. So Osoyoung's California scenario is unlikely, by the time the January & February thaws arrive, the trees are normally fully vernalized, ready to grow.
If you keep temperatures below 40 F, below 4 C, metabolism is so slow that if light were present, not much photosynthesis will occur. Mountain and extreme northern species such as some of the spruces, larches and a few others you actually need to keep temperatures below 36 F ( +2 C) to keep the trees fully dormant. Without light most healthy trees have plenty of stored carbohydrates to survive the dark spell. The problem is when your storage area warms up, and trees start to grow. Then the "In and Out dance'' begins, moving the tree into the sun for days above freezing and back into the shelter for nights below freezing.
As little as 24 to 48 hours above freezing will reduce the winter hardiness of a tree. For example blueberry flower buds, fully winterized, will survive -15 F (- 26 C) no problem. After 48 hours at 60 F (+16 C) they can be killed by 0 F (-18 C). From a study by MSU. This rapid loss of cold tolerance is shared by most species. This is why when the shelter hits 50 F for a day or two you need to do the dance to avoid frost or freezing the trees. The advantage of leaving trees outdoors, on the ground is that cold tolerance is lost at the same rate as the local native vegetation, which is usually slow enough to not have issues with late frosts.
However, I grow quite a number of trees from warmer climates, A zone 7 tree needs a winter chill, but won't survive my zone 5 winter, even if planted in the ground. So these are the trees I have to put in my unheated well house. I can usually hold them dormant for most of the winter. Problem is the ground thaws, warms up, usually 2 to 3 weeks before it is safe to start putting trees outdoors. My well house is ground temperature, so Japanese Maples and other warmer natured trees sprout a couple weeks early. These become my trees I have to shlepp in and out. I tried lights in the well house. It was a fail because the ceiling is low, the room is not a large volume of air, so lights bright enough to do any good warmed the well house up 10 or more degrees beyond ground temperature. Bad for the trees. So I keep them in the dark. Any trees that bud out and start growing, get ''schlepped'' in and out to get them sufficient light for growth.
Trees outdoors I don't have to worry about as much. There is wisdom in only growing trees that are native to your climate and or a climate that is a zone or two colder than your own. I have an anderson flat of eastern hemlocks that is spending its 3rd winter on top of my bench, right were it grew all summer. It looks fine.
Ryan Neil can be cavalier about leaving trees outdoors, he lives in Oregon, zone 7 or zone 8 - a very mild climate. Not hot, not cold.
Mike Frary lives in a beastly cold climate. He can't be cavalier. I live a couple hundred miles south of Mike, in a climate that is pretty similar to Vance Wood's area, though because I am so close to Lake Michigan, my spring is much cooler than people 3 miles west of me, and my autumn stays warmer, or at least frost free for nearly 4 weeks longer than people living 3 miles west of me. "Lake Effect'' is interesting. I've learned to rely on it, and it usually is reliable.
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Here in California, we have a very real and similar influence from proximity to the Pacific. When we moved here, the realtor told us - for every mile you are away from the ocean, expect your summer heat to be 1 degree warmer, and your winter cold to be 1 degree colder. This rule of thumb is pretty accurate for the first ten miles or so, at which point the prevailing wind direction (off the ocean = cool, off the desert = hot) has a greater impact. I live about 5 miles off the ocean - and it is amazing how temperate it can be just a little closer to the coast.
lol!
