Stop shivering! These 'secret' thermals offer warmth, not bulk — save up to 45%

These No. 1 bestsellers have almost 35,000 five-star fans on Amazon, so they must be doing something right. They're made from a stretchy polyester-spandex blend that's lined with cozy fleece and come in sizes XXS-3XL. Choose from 14 colors and three necklines — classic crewneck, scoop neck or V-neck.

"If you want something lightweight and thin that you can wear under your clothes no matter if you're going outside, or if you're working in an office and you're just cold-blooded, these are thin enough," said this shopper. "You can wear under anything and not feel too bulky at all, but the fabric is warm and retains body heat."

Looking for something a little sportier? This fleece-lined, moisture-wicking set of long johns is adventure-ready, with hidden pockets in the pants and flatlock seams that won't irritate your skin. It also comes in gray.

"They are nice and stretchy; the outer material helps the clothes you're putting on over them glide across and not get bunched up, and they aren't a super-thick material to where you feel like you can't move," a fan wrote. "I bought these for wearing under my motorcycle riding gear for those chillier mornings, and they have been wonderful! I'm no longer freezing halfway through my commute to work."

Crz Yoga makes some of our favorite comfy clothes on Amazon, so we'd expect the same quality from the brand's thermal underwear. The anti-static, anti-pilling fabric blend includes super-soft viscose. Choose from 12 solid colors in sizes XXS to XL.

"Great quality comparable to Helley Hansen and so soft. They feel slinky and thin but really keep you warm," this happy buyer said. "Also the cut is really cute."

Want a base layer that'll move with you? Whether you're heading for the ski slopes or out to shovel the driveway, this seamless, quick-drying set offers light compression similar to your favorite workout wear. Choose from eight colors.

"It feels like a second skin; I even forgot I was wearing it," said a buyer. "I used it to go hunting, temperature was about 10° with the windchill, and it kept me warm and comfortable all the time. I didn't get super-warm or sweaty."

If you're over polyester blends, behold these 100% mulberry silk beauties. They're ideal for sensitive skin — no itching or irritation — and the breathable knit is surprisingly warm when layered under clothing, fans report. These can even double as luxurious cooling PJs in warmer weather.

"These fit well and are so lightweight that it is easy to wear them under regular clothing," a buyer said. "They are about as thick as a layer of tights, but so warm. ... They don't get too warm when I'm indoors, but are very warm under clothes when I'm outdoors. They wash up easily in a lingerie bag in the washer. ... They will last for years if you take care of them."

If you want serious warmth without the bulk, merino wool checks all the boxes: It's soft, moisture-wicking, odor-resistant, breathable and ultra-durable. Choose from three thicknesses — this set is a versatile midweight — and sizes XS to XXL. 

"I took two sets of merino wool base layers on our trip to Iceland in November. ... Unfortunately, we didn't luck out with the weather; it was cold and rainy, with strong gusts of wind. I also packed several Omni-Heat thermals, but those made from synthetic materials were useless in Iceland's wet and cold conditions. The merino sets, however, kept me warm even when I was soaked. I wore both sets every day, and there was no odor. I'm very impressed and will definitely buy more."