Purpose: We investigated the effect of daily cold water immersion (CWI), during a typical training week, on parasympathetic activity and subjective ratings of well-being.
Methods: Over two different weeks, eight highly trained swimmers (4 men; 19.6 ± 3.2 y) performed their usual training load (5 d/wk, approx. 21 h/wk). Last training session of each training day was immediately followed by 5 min of seated recovery, in randomized order, with CWI (15°C) or without (CON). Each morning before the first training session (6:30 AM) during the two experimental weeks, subjective ratings of well-being (eg, quality of sleep) were assessed and the R-R intervals were recorded for 5 min in supine position. A vagal-related index (ie, natural logarithm of the square root of the mean of the sum of the squares of differences between adjacent normal R-R intervals; Ln rMSSD) was calculated from the last 3-min segment.
Results: Compared with CON, CWI effect on Ln rMSSD was rated as possibly beneficial on day 2 [7.0% (-3; 19)], likely beneficial on day 3 [20.0% (1.5; 43.5)], very likely beneficial on day 4 [30.4% (12.2; 51.6)] and likely beneficial on day 5 [24.1% (-0.4; 54.8)]. Cold water immersion was associated with a likely greater quality of sleep on day 2 [30.0% (2.7; 64.6)], very likely on day 3 [31.0% (5.0; 63.1)] and likely on day 4 [38.6% (11.4; 72.4)] when compared with CON.
Conclusion: Five minutes of CWI following training can reduce the usual exercise-induced decrease in parasympathetic activity and is associated with improved rating of perceived sleep quality.