Sleep apnea is a widespread disorder and is defined by the complete or partial cessation of breathing. Obstructive sleep apnea (OSA) is caused by an obstruction in the upper airway while central sleep apnea (CSA) is characterized by a diminished or absent respiratory effort. It is crucial to differentiate between these respiratory subtypes as they require radically different treatments. Currently, diagnostic polysomnography (PSG) is used to determine respiratory thoracic and abdominal movement patterns using plethysmography belt signals, to distinguish between OSA and CSA. There is significant manual technician interrater variability between these classifications, especially in the evaluation of CSA. We hypothesize that an increased body mass index (BMI) will cause decreased belt signal excursions that increase false scorings of CSA. The hypothesis was investigated by calculating the envelope as a continuous signal of belt signals in 2833 subjects from the MrOS Sleep Study and extracting a mean value of each of the envelopes for each subject. Using linear regression, we found that an increased BMI was associated with lower excursions during REM sleep (-0.013 [mV] thoracic and -0.018 [mV] abdominal, per BMI) and non-REM (-0.014 [mV] thoracic and -0.012 [mV] abdominal, per BMI). We conclude that increased BMI leads to lower excursions in the belt signals during event-free sleep, and that OSA and CSA events are harder to distinguish in subjects with high BMI. This has a major implication for the correct identification of CSA/OSA and its treatment.