Background: Various studies have linked the occurrence of cardiovascular events and low ambient temperatures as well as the morning blood pressure surge (MBPS). We hypothesized that low ambient temperatures produce a higher sympathetic change during the last rapid eye movement (REM) sleep transition and that this may play an important role in cold-related cardiovascular events. Methods: All experiments were carried out on 12 healthy male adults, aged 24.00. ±. 0.74. years, who participated in two experimental conditions randomly (<1. day apart): warm (23. °C) and cold (16. °C). Blood pressure (BP) was measured every 30. min for 24. h by autonomic ambulatory BP monitoring. The electroencephalograms, electrocardiograms, ambient temperature, near-body temperature, and physical activity were recorded by miniature polysomnography for 24. h. Results: The cold conditions resulted in: (i) higher MBPS than under warm conditions; (ii) significant and greater sympathetic index changes during the sleep-wake transition than during cover-to-uncover and supine-to-sit position tests; (iii) the non-REM-REM transition-related sympathetic elevation during the cold conditions being significantly higher in late sleep period than in early sleep period; (iv) at 1. h prior to morning awakening, the value of total power of heart rate variability changes being significantly negatively correlated with the changes of near-body temperature; and (v) significantly higher arousal index and shorter average interval of REM periods than in warm conditions. Conclusion: Cold exposure elevates the amplitude of MBPS and is associated with late sleep stage transition sympathetic activation, which might have important implications for cold-related cardiovascular events.