Flow is an optimal experience characterized by the perception of high challenges and high skills, positive affect, complete absorption in the activity carried out and intrinsic motivation. Although much research has examined the psychological features of flow, little is known about its biological underpinnings. The present study aimed at contributing to this gap by investigating the psychophysiological correlates of flow experience during daily routines. To this end, 15 university students took part in an experience sampling study, in which they provided real-time information on daily activities and associated experience while cardiac activity was monitored. After seven days of observation, 32 flow events were identified among 10 participants. A multilevel regression analysis revealed a significant correlation between optimal experience and specific cardiovascular indexes. In particular, the experience of flow was associated with increased heart rate and increased LF/HF ratio, suggesting relative sympathetic enhancement. These findings are in line with those obtained by previous related studies and indicate the feasibility of investigating physiological correlates of subjective experience in ecological contexts.