Table 1.

Description of the variables.

Variable	Definition
Heart rate variability
RMSSD (ms)	Average every 5 min of the RMSSD which is the square root of the mean of the union of the adjacent R–R intervals. Measured during sleep. This is a vagal heart control indicator (parasympathetic tone). The higher the RMSSD, the lower the physiological stress and higher health.
RMSSD 4h	Average minute by minute of the RMSSD during the first 4 h of sleep after the first half an hour of sleep.
Stress and recovery
% Stress	Indicates the percentage of time between the stress reactions within/for 24 h.
% Recovery	Indicates the percentage of time between the recovery reactions within/for 24 h.
Ratio stress-recovery	Ratio of the division between % of stress and % of recovery, where higher values mean higher stress levels in 24 h.
Stress Balance	The difference between the total time classified as recovery during sleep and the total time classified as stress during sleep, divided by the sum of total time classified as recovery during sleep and of the total time classified as stress during sleep. Values from 0.5 to 1 indicate a good recovery; values of 0 to 0.5 indicate moderate recovery and values of 0 to −1 indicate insufficient recovery.

RMSSD = root mean square of the successive difference.