Lately, I have woken in the night with my heart racing. This has been happening once or twice a week, sometimes lasting an hour, leaving me a bit light headed and feeling sick. I've ruled out too much caffeine, and wonder if anyone has experienced similar symptons? On most (but not all) occasions I have had a hard running training session the previous evening, but don't know if this is a pure coincidence. My heart rate is fine running. I'm a bit stressed at work, but nothing out of the ordinary. Any thoughts??