HR in the evening vrs in the morning?

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Can anyone help me understand - why when I swam the other evening same length of time (1 hour) same number of lengths (80) - my calorie burn was 511 and average heart rate was 137 with a maximum of 160. And this morning when I swam (1hour / 80 lengths) my calorie burn was 382 average heart rate was 119 with a maximum of 134. Is my heart rate just naturally lower in the morning?