Cryotherapy — which involves briefly exposing the body to extremely cold temperatures — continues to be a hot topic. Many of the studies on cryotherapy focus on the effects on exercise metrics, including muscle soreness or muscle growth, both of which can be inhibited by cold exposure.

However, the summarized study is notable because it investigated some of the other effects of cold, specifically sleep and mood. This study suggested that cold may have improved these outcomes. Other studies have also examined the effects of cold therapy on sleep, nervous system activity, cognitive function, and mental health.

Potential health effects of cold therapy

Sleep: In a 2019 RCT by some of the same authors as in the summarized study, air cryotherapy reduced movements during the night (one proxy for sleep quality), with a large effect size, in professional soccer players.^[1] However, in 2 other RCTs featuring field hockey players and cyclists, there were no effects of postexercise cold water immersion on subjective sleep quality, perceived stress, or fatigue.^[2]^[3]

Nervous system activity: In another 2019 RCT by some of the same authors, the researchers also found improved sleep quality and an increase in parasympathetic (rest and digest) nervous system (PNS) activity during deep sleep, as measured by the proxy of heart rate variability (HRV).^[4]

The PNS plays many roles, from decreasing heart rate and blood pressure to increasing HRV and digestive function (e.g., the release of digestive enzymes). Accordingly, the sympathetic nervous system (ANS), can increase heart rate and blood pressure, decrease HRV, and decrease digestive function.

A 2024 meta-analysis of 17 RCTs reported moderate benefits of cold water immersion, compared to other recovery techniques, for increasing PNS activity post-exercise.^[5]

Additionally, a 2024 meta-analysis of 27 studies by one of the same authors as the summarized study examined HRV after a variety of cold exposures and also found overall enhancements to PNS activity. However the authors did note a small increase to average blood pressure, perhaps due to increased activation of the ANS.^[6] Increased ANS activity has been noted elsewhere, as the researchers in a 2016 RCT noted higher blood pressure and lower HRV in the early morning from cold exposure.^[7]

In another 2024 crossover trial by some of the same authors as the summarized study, cold therapy 1 hour before bed was more effective than cold therapy 3 hours before bed after a running session for increasing PNS activity and lowering core body temperature during sleep.^[8]

Mental health: A 2021 meta-analysis of 10 studies noted beneficial effects of whole-body cryotherapy on mental health, including improvements in depression symptoms, though there was a high degree of heterogeneity in these studies.^[9]

Furthermore, a 2008 nonrandomized clinical trial of daily air cryotherapy sessions (2–3 minutes at approximately –200℉, or –130℃ daily for 15 days) found that the treatment reduced anxiety symptoms compared to a control.^[10]

Cognition: In the very short term, cold exposure tends to impair cognition. In a 2021 systematic review, investigators looked at 18 clinical trials that tested the cognitive effects of cold exposure via cryotherapy or cold water immersion.^[11] Generally, cold exposure worsened aspects of cognitive function, like attention and memory, when testing occurred during or within approximately 60 minutes of the cold exposure. Detriments were less consistently observed when testing began 90 minutes or longer after cold exposure.

In the longer term, the effect of cold therapy on cognitive function is less clear. In a controlled trial from 2006, repeated cold exposure over 10 days had no apparent effect on cognitive function.^[12] In a 2021 RCT, participants with mild cognitive impairment (average ages of 66–67) underwent 10 air cryotherapy sessions (2–3 minutes at approximately –200℉/–130℃ every weekday for 15 days) and experienced improvements to mood, immediate recall, and psychological orientation, despite a counterintuitive decrease in serum BDNF.^[13]

Are there risks with cold therapy?

A 2023 review of adverse effects of whole-body cryotherapy noted that there have been reports of increases in blood pressure, headache, and urticaria in some participants. Rarely, there were even cases of serious cardiovascular events, perhaps from this short-term increase in blood pressure.^[14]

The largest risk of cold water immersion is losing consciousness and drowning when cryotherapy is performed alone.^[15] This is why water cryotherapy is highly recommended to be done only under supervision.

Are water and air cold therapy comparable?

Water conducts more thermal energy than air, which is why 40℉ (–4℃) water feels colder than 40℉ air, and why air cryotherapy is done at such extreme temperatures. With these extreme temperatures, is air cryotherapy comparable to water cryotherapy? Trials show inconsistent results.

The researchers in a 2019 RCT found no differences between air and water cryotherapy for improving delayed onset muscle soreness, muscle swelling, and the recovery of power production 72 hours after exercise. There were some sites on the body where the skin temperature was lower in the water group than in the air group, but the results were inconsistent.^[16]

A 2019 randomized crossover trial showed a stronger effect of water, compared to air. The researchers had the participants perform hamstring exercises and tested recovery after 0, 24, 48, and 72 hours. They found a somewhat consistent benefit of water cryotherapy compared to air cryotherapy: improved recovery of power production recovery at 72 hours, reduced soreness at 24 and 48 hours, and lower levels of muscle damage at 24 hours (creatine kinase), though the soreness findings were of borderline statistical significance.^[17]

In contrast, a 2020 randomized crossover study examined air cryotherapy, cold water therapy, contrast hydrotherapy (alternating between warm and cool water), and a control.^[18] The researchers found that the air cryotherapy consistently improved recovery (recovery from vertical jump height), soreness, muscle damage levels, and inflammation (C-reactive protein) more than water cryotherapy and the control, at up to 72 hours after exercise. There were no other consistent differences between groups.

One possible reason for the discrepancy between these trials is that the 2020 trial used warmer water (59℉, or 15℃) than the 2019 trials (50℉, or 10℃).

It is difficult to discern how air cryotherapy compares to cold cryotherapy. Water at 50℉ might be more effective or similar to air cryotherapy at 166℉ to 211℉ , but 50℉ water is definitely easier to access!

Conclusion

Ultimately, cryotherapy, whether involving water or air, shows some evidence for improving aspects of mental health, but it has weak evidence for improving cognition and sleep. People who are regularly undergoing resistance training should also weigh these benefits against the generally small decreases in muscle growth and muscle strength gains observed with cryotherapy.

Does cold therapy improve sleep and mood? Original paper