It was our impression that the respiratory parameters in obstructive sleep apnea (OSA) worsened as the night progressed. To confirm this, we review polysomnographic studies from 66 patients with apnea-hypopnea indices (AHI) of 40 to 125 events per hour, dividing bed time into equal quartiles. As the night progressed, the mean apnea duration (MAD) increased from 27.2 s to 34.6 s (p < 0.0001), mainly from increases during NREM sleep. The proportion of time spent in apnea increased from 54 to 71% (p < 0.0001) due to increases in both MAD and the proportion of REM sleep (from 2.8 to 14.7% of the total sleep time). The AHI did not change significantly between quartiles. Even though preapneic oxygen saturation did not change and apnea duration increased as the night progressed, the end-apneic saturation did not decrease, hence the rate of oxygen desaturation decreased. Also, it was found that patients with an AHI greater than 65 events per hour increased their proportion of time spent in apnea significantly more than those with an AHI smaller than 65, as the night progressed. In the patients with an AHI greater than 85, this was due to both an increase in MAD and AHI. In conclusion, in patients with an AHI greater than 40 events per hour, the severity of apnea increased as the night progressed due to lengthening of MAD, increased proportion of REM sleep, and in the most severe patients, also an increase in AHI. Even though the exact pathophysiologic mechanisms for the observed changes are unknown, a decrease in respiratory muscle effort with consequent decrease in oxygen consumption may explain both the lengthening of the apneas and the decrease in the rate of oxygen desaturation.