The Effect of Water Temperature on Heart Rate Variability and Cognition

Heart rate variability (HRV) is a widely recognized physiological marker that reflects the dynamic interaction between the cardiovascular and autonomic nervous systems. HRV represents millisecond-level fluctuations in time between consecutive heartbeats. It is a key indicator of autonomic nervous system balance, with higher values indicate better recovery, fitness, and stress resilience. Previous research has shown that HRV decreases under mental workload, including during tasks that require attention, inhibition, or executive function.1-3 Studies also show that temperature exposure affects autonomic regulation: cold increases sympathetic drive, while warm water supports parasympathetic activation.4-6 Very few studies combine thermal exposure with cognitive testing, and no controlled trials have investigated whether water temperature modulates HRV responses specifically during cognitive demand.

Inclusion criteria

• Healthy women free of chronic disease (by self-report)
• 18-30 years of age

Exclusion criteria

• medication/drug use that has not been consistent for the previous 3 months or will be discontinued during the trial
• any acute illness
• regular use of cigarettes or marijuana
• pregnant or lactating women
• not fluent in English
• not willing to attend two in-person lab visits in downtown Phoenix and follow study protocol (including abstaining from caffeine in the 12 hours prior to testing)
• not currently competing in sports or unusually high-intensity exercise protocols (e.g., high-intensity exercise greater than 30 minutes most days of the week)