shelleycat
About 4 am is when the body really starts cleansing - liver, kidneys, everything. That would certainly affect the heart rate, especially with the extra salts and magnesium in your system, which would make the kidneys work harder to re-establish PH balance.
Outside of drinking more water throughout the day of the cleanse there isn't much you can do. It's not a bad thing or something to worry about really, it just means your body is working very hard, harder than usual, to cleanse. :)