Cold therapy benefits, brown fat, and recovery with Dr. Susanna Søberg

Podcast episode originally published on January 31, 2023

Cold therapy can raise alertness, activate brown fat, and change how your body responds to stress, but the useful questions are practical: how cold is cold enough, how long is enough, and when should you do it? In this episode of the WHOOP Podcast, Global Head of Human Performance, Principal Scientist at WHOOP Dr. Kristen Holmes speaks with Dr. Susanna Søberg, a metabolism researcher with a PhD from the University of Copenhagen and founder of the Soberg Institute, about what cold exposure does to the nervous system, metabolism, sleep, and recovery. Their discussion moves past hype and into application, including the 11-minute weekly threshold Søberg studied, the role of brown fat, and why timing cold around workouts can change the outcome.

For the full conversation on winter swimming, brown fat, and the Søberg Principle, listen to Episode 207 of the WHOOP Podcast.

How does cold therapy change the body in the first few minutes?

Cold therapy changes the body immediately. Cold receptors in the skin send an emergency signal to the hypothalamus, which raises sympathetic activity, constricts blood vessels near the skin, and shifts blood toward the core to protect vital organs.

Søberg explains that this early phase is the classic cold shock response. Heart rate rises, blood pressure rises briefly, and stress hormones go up. At the same time, full-body submersion adds another layer. When you enter cold water, especially up to the neck, you also trigger the diving reflex, which increases parasympathetic activity through the vagus nerve. That combination helps explain why cold can feel intensely activating at first and then noticeably calming once breathing settles.

This matters for interpretation. A cold shower and a cold plunge do not create the same signal. A shower can strongly stimulate the sympathetic nervous system, while immersion is more likely to bring both sympathetic and parasympathetic branches into the same session. It also helps explain why WHOOP data may look different when you compare a first week of cold exposure with a month of steady practice.

Holmes also raised an important caution that stays relevant outside the episode. If you have heart disease, symptoms that raise concern during cold exposure, or unregulated high blood pressure, cold plunging is something to discuss with a clinician before trying.

As Søberg described the early minutes in the water:

"Norepinephrine will go up by 2.5-fold within just a few minutes as soon as you go into your ice bath. [...] It's only 9 to 12 millimeters mercury, so it's very little actually that science shows that the blood pressure goes up, and it's very brief that it goes up."

What you should take away

• Cold receptors in the skin trigger a fast stress response that begins before you have spent much time in the water
• Full-body cold immersion can activate both sympathetic and parasympathetic pathways because submersion adds the diving reflex
• Brief increases in heart rate and blood pressure are part of the expected cold shock response, but people with cardiovascular concerns should be cautious

Søberg walks through the cold shock response and diving reflex in more detail in the full episode over on Youtube.

How does cold exposure activate brown fat and metabolism?

Once the early shock response is clear, the next question is what the cold is doing metabolically. Søberg's main point is that brown fat is a heat-producing tissue, and cold is one of the fastest ways to switch it on.

Brown adipose tissue is different from the white fat most people think of first. It is rich in mitochondria and burns fuel to generate heat. That is why researchers became so interested when imaging studies confirmed that adults still have cold-activated brown fat, not just infants. Two landmark papers in The New England Journal of Medicine helped reestablish that point in adults: Identification and importance of brown adipose tissue in adult humans and Cold-activated brown adipose tissue in healthy men.

Søberg argues that the metabolic value goes beyond heat production. In the episode, she described how repeated cold exposure can improve glucose handling and insulin sensitivity because activated brown fat pulls fuel from circulation. That is consistent with later human work, including short-term cold acclimation improving insulin sensitivity in people with type 2 diabetes.

Her own research adds a practical threshold that makes the topic easier to apply. In a controlled comparison of adapted winter swimmers and matched controls, Søberg found that a relatively small weekly dose of cold immersion was linked with brown fat activation, faster glucose handling, and improved insulin sensitivity in healthy participants. WHOOP has also covered the broader physiology behind cold therapy benefits if you want a separate explainer on the mechanism.

Søberg summed up the dose from that work in one of the clearest lines from the episode:

"We saw that 11 minutes of cold water immersion per week, not in one time, but divided on 2 to 3 days, actually activated our brown fat and also increased our insulin sensitivity in these healthy subjects."

What you should take away

• Brown fat is a heat-producing tissue that helps the body generate warmth by burning fuel
• Cold exposure can support metabolism because activated brown fat uses glucose and fat from circulation
• Søberg's research suggests that 11 total minutes of cold water immersion per week, split across 2 to 3 sessions, was enough to produce measurable changes in healthy participants

If you want Søberg's longer explanation of brown fat, insulin sensitivity, and why adults still have this tissue, go to the full episode over on Youtube.

How long should you stay in cold water, and how cold should it be?

You do not need extreme sessions to benefit from cold exposure. Søberg's framing is simple: stay in long enough to get through the initial shock and create adaptation, then get out before the cold keeps driving core temperature down.

Adaptation changes that timing. Someone new to cold exposure may spend more of the session in a pronounced shock response, while someone adapted to winter swimming may settle faster. Søberg put the cold shock window at roughly 1 to 2 minutes, with the exact timing depending on experience and adaptation. That is one reason a short, consistent practice beats rare heroic efforts.

Water temperature matters, too, but not in the way social media often suggests. Søberg said cold water can reasonably be defined as about 15 degrees Celsius, or 59 degrees Fahrenheit, and below. She also emphasized that you do not need near-freezing water to activate a useful response. Even milder skin cooling, such as going outside lightly dressed in cool weather, can still contribute to brown fat activation.

A second useful point from the episode is variation. Repeated exposure to exactly the same temperature may reduce the novelty of the stressor. Søberg prefers shifting the temperature within a safe range instead of chasing one fixed number every time. People who pair heat and cold can also compare how each feels in practice. WHOOP has a related breakdown of contrast therapy if you want a separate look at alternating hot and cold.

Søberg described the adaptation window this way:

"The 1 minute is just approximately because we cannot really take everyone under one and say it's always 1 minute because it's not. [...] Between 1 and 2 minutes depending on how adapted you are."

What you should take away

• Most people do not need long cold plunges, because the useful stimulus comes early in the session
• The cold shock response often lasts about 1 to 2 minutes, but new participants may need more time to settle their breathing
• Water around 15 degrees Celsius, or 59 degrees Fahrenheit, and below is cold enough to count as cold exposure in Søberg's framework
• Changing the temperature across sessions can keep the stressor fresh without forcing longer exposures

When should you do cold therapy for energy, sleep, or a steadier routine?

The best time for cold therapy depends on the effect you want. Morning cold often feels stimulating, afternoon cold may fit better if you want some distance from bedtime, and nighttime cold remains highly individual.

Søberg was careful here. She said the research on precise circadian timing is still thin, so she did not give one universal rule. Instead, she reasoned from physiology. Cold can increase cortisol and catecholamines, which is why some people find a cold shower useful when they feel sluggish in the morning. Others report better sleep if they use cold earlier in the day and let the alerting effect wear off by night.

Holmes added an interesting piece of internal WHOOP analysis from people who logged ice baths. On days when members recorded an ice bath, average sleep consistency increased by 2.6 percent. That does not prove cause and effect, but it does suggest cold exposure may affect more than one behavior at a time. It may change how alert you feel, how routine-driven your day becomes, or how likely you are to protect your sleep window.

Søberg also said adapted winter swimmers in her work showed a stronger morning cortisol rise than matched controls, which she interpreted as a possible sign of a stronger wake-up signal. WHOOP has also published observations on sleep and recovery during cold-weather isolation training, which is a different setting but a useful reminder that cold stress and recovery interact in ways that deserve context.

When Holmes asked about morning effects, Søberg answered with appropriate caution:

"The winter swimming group adapted to cold water, they had this spike of cortisol in the morning, which was higher than the control group, which could indicate that they have a better metabolism, they have a better circadian rhythm, and they have a better waking up, you can say, clock in the body to get up and get started with the day."

What you should take away

• Morning cold may be useful when the goal is alertness and a faster start to the day
• Afternoon cold may suit people who want the stimulus without pushing it too close to bedtime
• WHOOP data shared in the episode showed a 2.6 percent increase in sleep consistency on days members logged an ice bath
• Timing is individual, so the best approach is to test cold exposure at different points in the day and compare the result

Holmes and Søberg spend more time on timing, sleep, and circadian questions in the full episode over on Youtube.

Should you use cold therapy before or after workouts?

Use cold before training if the main goal is alertness and a strong training feel. Be more selective with cold right after workouts if muscle growth is the goal, because postworkout cold may reduce some of the inflammatory signaling that supports hypertrophy.

This was one of the most practical parts of the episode. Holmes described feeling unusually sharp and capable when she uses cold before a session, and Søberg agreed with the sequence. Her reasoning was straightforward. A short cold exposure can sharply increase catecholamines and make you feel switched on, while putting the cold after training may blunt muscle soreness and reduce pain, but also reduce some of the inflammatory process you want if you are trying to build muscle.

That does not make postworkout cold useless. It changes the tradeoff. If you are in a competition block, playing games on consecutive days, or simply trying to reduce soreness so you can train again soon, postworkout cold may still make sense. If you are in a heavy strength phase and care most about hypertrophy, preworkout cold or cold on separate days may fit better.

That split mirrors how WHOOP members can think about the practice. The same cold plunge can serve a different purpose depending on whether you want sharper training, lower soreness, or better day-to-day adherence. WHOOP has also explored how cold-weather training changes perceived effort, heart rate, and recovery demands in running.

Søberg's simplest description of preworkout cold was also one of the most memorable:

"It is actually an energy boost that is much greater than drinking coffee or energy drinks or whatever before you go to your workout."

What you should take away

• Cold before training may help with alertness, motivation, and perceived energy
• Cold right after lifting may reduce soreness, but it can also reduce some of the inflammation tied to muscle-building adaptation
• Postworkout cold is often easier to justify during competition periods or dense training blocks when next-session readiness matters most
• Separating cold exposure from key strength sessions is a practical middle ground for people who want both adaptation and recovery support

How can you make cold exposure easier to stick with?

The best way to stick with cold exposure is to treat it as a skill, not a test of toughness. Start with smaller doses, use breathing to get through the shock, and build up slowly enough that the practice remains repeatable.

Søberg's coaching advice was practical. You can begin with hands in cold water, a cold face rinse, or a short cold finish at the end of a shower. The goal is to learn how your body reacts and how quickly you can regain control of breathing. Once breathing steadies, the session has usually already done useful work.

Her preferred cue was simple: breathe deep, low, and slow. That is how she teaches people to move from panic toward control. In the episode, she also stressed that no one needs to master this on day one. Ten seconds is a start. Thirty seconds is a start. Consistency is the point.

Søberg also spent time on the setting. Outdoor cold can be easier to repeat when it comes with a walk, open water, daylight, or other routines that feel good on their own. She referenced research on forest bathing and time in nature lowering stress hormones [LINK NEEDED], which helps explain why winter swimming can feel different from sitting alone in a tub in a garage. WHOOP has looked at related brain-health themes in How exercise improves cognitive function and longevity with Dr. Tommy Wood, and the overlap here is clear: stress dose, routine, and environment all shape the result.

Søberg's shortest coaching line may be the most usable one:

"So deep, low, and slow, I always say."

What you should take away

• Cold exposure becomes easier when you build it like a habit and not like a dare.
• Breathing control is one of the fastest ways to move through the first wave of panic in a cold shower or plunge.
• Short entries, cold shower finishes, and skin-only exposure are valid starting points.
• Nature, daylight, and routine can make cold exposure easier to repeat and easier to enjoy.

For Søberg's advice on building tolerance, using breathwork, and handling the mental barrier, listen to the full episode over on Youtube.

The bottom line

• Cold therapy triggers a fast whole-body response that starts with skin receptors, sympathetic activation, and blood shifting toward the core
• Cold water immersion can activate both sympathetic and parasympathetic pathways because submersion adds the diving reflex
• Brown fat is a heat-producing tissue in adults, and repeated cold exposure can support metabolism by increasing fuel use for heat production
• Søberg's research found that 11 total minutes of cold water immersion per week, split across 2 to 3 sessions, was enough to activate brown fat and improve insulin sensitivity in healthy participants
• Water does not need to be extreme to count as cold, because Søberg defines cold water as roughly 15 degrees Celsius, or 59 degrees Fahrenheit, and below
• Cold before training may improve alertness and perceived energy, while cold right after lifting may reduce soreness at the cost of some muscle-building signaling
• The best timing for cold therapy depends on the goal, with morning often feeling more stimulating and later sessions requiring personal testing around sleep
• Breathing control, gradual exposure, and consistent repetition make cold therapy safer and easier to sustain than one-off extreme plunges

Frequently asked questions about things discussed in this episode

How does WHOOP help you track cold therapy?

WHOOP lets you log cold showers, ice baths, and related recovery work so you can compare those habits against trends in Recovery, Sleep, resting heart rate, and HRV over time.

What does WHOOP show after an ice bath or cold shower?

WHOOP shows how your body responded after the fact, rather than telling you that one session was automatically good or bad. The useful pattern is whether cold exposure lines up with better sleep consistency, different next-day Recovery, or changes in resting heart rate and HRV across repeated sessions.

How can WHOOP help you compare cold before training versus cold after training?

WHOOP helps by giving you a repeatable way to compare the timing against training load and recovery markers. Logging preworkout cold and postworkout cold separately in WHOOP Journal can make it easier to see which setup fits your Strain, soreness, sleep, and next-day readiness better.

What does WHOOP tell you about sleep after cold therapy?

WHOOP can show whether cold exposure is associated with changes in sleep consistency, total sleep, and next-day Recovery. Holmes shared internal WHOOP analysis in this episode showing that sleep consistency rose on days members logged an ice bath.

How does WHOOP help you build a safer cold exposure habit?

WHOOP helps by giving you a record of frequency and timing, which makes it easier to progress gradually instead of guessing. A written log inside WHOOP Journal can also show whether longer or later sessions are disrupting sleep or leaving you feeling run down.

What does WHOOP Journal do for cold therapy experiments?

WHOOP Journal gives you a simple way to tag the exact behavior you are testing. That makes it easier to compare cold showers, plunges, timing, and session length against the recovery and sleep patterns that follow.

Logging cold showers or ice baths in WHOOP Journal can help you see whether a few controlled minutes in the cold improve your sleep consistency, next-day Recovery, or training readiness.