Pain exhibits temporal variations in intensity due to multiple factors, including endogenous neuroendocrine and various external influences that vary over the 24 h. Also, medications can vary in potency and/or toxicity according to the time when they are administered. However, there is no consensus among studies regarding the 24-h pattern of analgesia during labor. Taking into account the time-of-day when labor analgesia is administered, this study aimed to answer two questions: (i) Is there diurnal variation in visual analogue scale (VAS)-rated pain relief and duration of intrathecal analgesia in patients undergoing labor analgesia? (ii) If there is, what is the influence of the duration of labor on the diurnal variation of the level of pain relief and duration of intrathecal analgesia? This prospective cohort included 41 healthy, nulliparous women in the first stage of labor undergoing spinal-epidural (CSE) analgesia using fentanyl combined with bupivacaine. Subjects had an epidural catheter fitted for additional, patient-controlled analgesia (PCA) if their pain relief was unsatisfactory. The number of VAS assessments (n = 558) was divided into six time periods of the 24 h. The adjusted coefficient of determination (r(2)), the proportion of the variance explained by the association between the duration of labor and the temporal pattern of the outcomes variable, was 58% (r(2) = 0.58) for pain relief and 44% (r(2) = 0.44) for duration of intrathecal analgesia. The peak effect of labor analgesia occurred between 02:00 and 05:59 h. However, the duration of intrathecal analgesia showed two peaks, i.e., at approximately 00:00 and approximately 12:00 h. These results demonstrate that labor analgesia achieved by fentanyl combined with bupivacaine shows a diurnal pattern in pain relief and duration of spinal analgesia. However, part of these temporal patterns was explained by the association with duration of labor.