Complex sentence: This study has indicated that a large decline in blood pressure occurs during the daytime sleep-onset period only when sleep is expected however when subjects rest in a supine position, the same reduction in blood pressure is not observed.
Split into simpler sentences: This study has indicated that a large decline in BP occurs during the daytime sleep-onset period only when sleep is expected. However, when subjects rest in a supine position, the same reduction in BP is not observed.