Got my wee sprouts coming up vigorously, and I want to get them out and exposed to the wind (still in trays) and sunlight some over the next few weeks to help them get thicker and tougher. This is not exactly hardening off--they're currently spindly because I forgot to turn on my grow light for a day or two right after germination--more of an experiment. We're expecting temp ranges from 45-65 over the next few days. I know it's plenty warm at the top of that range, but will seedlings be harmed by mid 40s? I can easily bring them in at night or whenever it gets too cold. What do y'all think? Is it worth the risk?

Be cautious with the wind. I take my seedlings outside but shelter them from the wind as much as I can or they fall over and damage the stem. A very light breeze is all you need.

Also be cautious with cool temps at night. I bring mine inside at night or I risk damping off.

I'm with Zeuspaul on that one. Tomatoes less than 6 inches-- I take them out side during the day, and leave them covered. Moisture is held in, by a clear top container lid. Generally my tote containers have 6 inch sides. Sometimes my 2 leaf are in high raised cover lasagna pans. If it is going to be very hot, they might bake covered. So might just expose them to direct sunlight early, and later in the day.
Growth lights are very helpful. But will not prevent from you having long thin weak leggy tomato plants. If you want a big strong stem? Only the sun can do that. Just 3 hours of early or late sun works wonders. Mid-day sun can be brutal-- even killing. Complement your plants with your growth lights.. PS. The problem you face is once a plant is programed to take off, and grow high and leggy reaching for sunlight. It is hard to deprogram that plant-- sometimes impossible. This woman shows a solution to leggy plants.

I wouldn't rush it yet. Especially if you have a grow light, i would take advantage of that fact and wait a little more. Even going as far as to repot the spindly plants deeper before taking outside to get thrashed by the wind.

PS
Just watched the video above and that is the method i use. I can vouch for its success. The only difference I had was i used a "Q-Tip" in fear of damaging the plants. I think my fear was unwarranted

I would let them get several sets of true leaves before exposing them to mid 40s. A six week old tomato can take it if you don't disturb the roots, but if you're really at the sprout stage you're taking a chance with sun wind and cold stress all together. 55-65 F should be fine, but bring them in if it's dropping below 50 F, I would say.

TxTravis: Tomato seedlings hereabouts get blown for a few hours on warmer days. Use a whisper fan salvaged from an old computer.