What's the meaning of otherwise in this sentence?

Even a fifteen-minute burst of light in otherwise sustained darkness can reset an animal's circadian rhythm.

  • Have you looked up the word in a dictionary? Please edit your question to add what that told you, and what remains unclear to you. – Janus Bahs Jacquet Mar 2 '19 at 20:32
  • I've looked up the word in Longman dictionary, but I didn't get the meaning of that word in context. Hguler answered my question. – Mike Mar 3 '19 at 8:26

What it means is that darkness is sustained except for the fifteen-minute burst of light. If the word "otherwise" were left out, there would be potential confusion because darkness is not being sustained during the fifteen-minute burst of light.

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