The central argument here is simple and important: temperature protocols developed primarily on male subjects don't transfer cleanly to female physiology. Dr. Stacy Sims has been making this case for years, and this conversation with Huberman distills it well. Women are not small men. Their hormonal architecture, thermoregulatory responses, and stress tolerance windows are different — and those differences matter when you're deciding how cold, how hot, and for how long.
The 16°C threshold for cold water is the number that stuck with me. Not ice cold. Not the 10-12°C plunge that gets celebrated on social media. Around 55-56°F. That's cool enough to trigger the dopamine cascade and the metabolic response, but not so extreme that you're inducing the kind of vascular constriction that overshoots the benefit. The stress response should prime you, not overwhelm you.
In the knowledge base, there's a 2025 paper on acute Finnish sauna heating and cold water immersion that maps exactly what happens at the tissue level during contrast protocols. Cutaneous blood vessels constrict rapidly when you enter cold water after heat, normalizing core temperature and enabling safe repetition of cycles. The key word is "safe repetition." The protocol works because the body can recover between cycles. Push the cold temperature too low, and you extend the recovery time beyond what's adaptive.
There's also a paper on cold environment exposure and female reproductive health that sits in the knowledge base, and it reinforces Sims' caution around extreme cold for women. The reproductive system is particularly sensitive to acute thermal stress — not because women are fragile, but because the hypothalamic-pituitary axis that governs hormonal regulation is also deeply involved in thermoregulation. They're not separate systems. They talk to each other constantly.
Huberman and Sims broadly agree on heat. The sauna data is robust across sexes — cardiovascular adaptation, heat shock protein activation, growth hormone release, serotonin pathway engagement. The gut-serotonin connection Sims raises is elegant: 95% of serotonin is produced in the gut, the hypothalamus uses serotonin to regulate body temperature, and heat exposure stimulates this pathway directly. For women navigating perimenopause or managing hot flashes, this isn't a side effect — it's the mechanism.
The more interesting divergence is around infrared. Sims is dismissive of infrared sauna for this purpose, and she's right for a specific reason: infrared heats the skin but not the core. The adaptations you're after — cardiovascular, hormonal, metabolic — require core temperature elevation. A traditional Finnish sauna at 80-90°C delivers that. Infrared at 50-60°C largely doesn't.
If you're a woman building a contrast protocol: start with heat. Traditional sauna, 15-20 minutes, enough to genuinely sweat and feel your heart rate climb. Then cool water at around 16°C — a cold shower or a plunge that's brisk but not brutal. The contrast is the protocol. You don't need to suffer to adapt.
If you're pregnant, take Sims' advice at face value: consult someone who actually knows. Moderate heat — yoga in a warm room, a warm bath — can support vascularization. Extreme cold is a different stress altogether, and the first trimester especially is not the time for experimentation.
The male fertility note at the end is easy to overlook, but it reveals something important about how temperature and biology intersect at the reproductive level. Cooling the testicles increases testosterone. Heat near the testes decreases sperm viability. These aren't edge cases — they're fundamental biological signals. Temperature isn't just a wellness variable. It's a reproductive signal. Your body is constantly reading thermal cues and adjusting hormonal output accordingly. That's true for both sexes, in ways we're still mapping. Which is precisely why "just get in the ice bath" is not a protocol. It's a starting point that requires refinement based on who you actually are.