If you need a nap during the day, it may be that you're not getting enough quality sleep during the night. There are many factors you need to consider, including your environment (bedroom), your lifestyle, and your bedtime routine.
Make sure there is complete darkness in the room. Block out light even from a clock or cell phone. Try focusing on something which doesn't require a lot of though; like counting backwards or tightening and loosening your muscles.
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