© 2019, International Heart Journal Association. All rights reserved. Sudden unexplained nocturnal death syndrome (SUNDS) is prominent among northeast Thai men. This study tests the hypothesis that Thai men with positive family history of SUNDS display abnormal diurnal, autonomic nervous system responses to stress. Healthy northeast Thai men (20-49 years old) lived in the same rural area were divided into two groups based on their positive (PF) or negative family (NF1) history of SUNDS. A second control included Thai men with an NF history of SUNDS from a non-endemic area (NF2). All data were collected at 4:00-6:00 AM (nighttime) and 4:00-6:00 PM (daytime). All three groups displayed nighttime decreases in mean arterial pressure, heart rate, and blood glucose. Furthermore, all subjects displayed similar glucose tolerance and electrolyte balance. The tachycardic responses to a four-minute step test were similar among groups in the daytime, but the nighttime responses were significantly blunted in the PF group compared to either control group (about 20 bpm less). Tachycardic responses to a cold pressor test tended to decrease more during the nighttime in the PF compared to NF1 and NF2 groups, but the difference was not significant. Arterial pressure responses to the exercise were similar among the three groups during the nighttime, whereas in the NF2, daytime mean arterial pressures increased more than those in the other groups. The present data suggest that Thai men with a PF history of SUNDS display blunted sympathetic nervous system responses to stress during the nighttime, a potential factor that may trigger cardiac arrhythmias and contribute to SUNDS.