Endel's sound personalization algorithm is built around the curve of ultradian and circadian rhythms—universal biological energy cycles. The 'energy' hints you see on the main app screen show the current phase of the ultradian rhythm or rest-activity cycle. It guides your vital performance day and night and ensures that your energy refills every 110 minutes. Ultradian rhythms are a part of our 24-hour sleep-wake cycle—the circadian rhythm hardwired to our bodies. It regulates the secretion of th