Final answer:
Ray is likely experiencing difficulty sleeping at night due to a disruption in his circadian rhythm after switching from night shifts to day shifts.
Step-by-step explanation:
Ray's present difficulty in getting sleep at night is most likely due to a disruption of his normal circadian rhythm. Circadian rhythms are biological cycles that occur over approximately 24 hours, mainly influenced by light and darkness in an organism's environment. Ray's body has become accustomed to being awake at night due to working night shifts for an extended period, which misaligned his internal clock with the typical day-night cycle. Now that he works days, his circadian rhythm is out of sync with his new schedule, causing difficulty in falling asleep at the usual nighttime hours.