By definition, ORP in full wakefulness is very close to 2.50, and there is little variability in ORP among the ten 3-second epochs that constitute a conventional 30-second epoch. As the subject begins descent into sleep, theta activity starts appearing in some of the 3-second epochs (microsleep or sleep intrusions). The lower ORP assigned to these sleep intrusions reduces the average ORP of the 30-second epoch. The decrease in average ORP of the epoch depends on how many sleep intrusions occur and their duration and extent of decrease in ORP.
Scoring does not change from wake to sleep immediately as these intrusions appear. Rather, epochs continue to be scored as Wake until the sleep-like activity exceeds 15 seconds, which typically happens when the average ORP of the epoch decreases below 1.75 (1). It follows that in every PSG there will be wake 30-second epochs with very high ORP (full wakefulness) and others with intermediate values between 1.75 and 2.50, which occur during the transition to sleep. The overall average of all wake epochs is the weighted average of epochs in full wakefulness and those in transition to sleep.
It follows that excessive wake time (low sleep efficiency) associated with high wake ORP (e.g., >2.15) indicates that most wake time is spent in full wakefulness with little attempt at sleeping, and suggests low sleep pressure (hyperarousal state, circadian misalignment, poor sleep hygiene, short sleeper). Conversely, excessive wake time associated with low ORPW (e.g., <1.95) suggests frequent attempts at sleep (decreases in ORP) that are reversed before succeeding in converting the stage from wake to sleep. These may (Figure below), or may not, be associated with brief respiratory events.
Figure: Recurrent respiratory events as the patient drifts to sleep. Note the reduction in ORP during the apneic periods and the resulting low average ORPW.
The circumstances that lead to excessive wake time with high ORPW (low sleep pressure) generally do not lead to excessive sleepiness (EDS), whereas those that lead to excessive wake time with low ORPW (unsuccessful attempts at sleep) are often associated with EDS. Low ORPW was associated with increased likelihood of EDS and poor subjective sleep quality, and vice versa, in two large independent studies (2). One useful application of ORPW is, therefore, to help explain the mechanism of excessive wake time (low sleep pressure vs. a disorder that interferes with progression to sleep). In the latter case, and in the absence of such disorders in the PSG (e.g., OSA, PLM disorder), investigation of other disorders that may interfere with sleep progression (e.g., pain disorders, central disorder of slow progression to sleep; see ORP-9 below).
Another important application of ORPW is to help explain why average ORP during non-REM sleep (ORPNR) is high in a patient. For the same reasons, high ORPNR associated with high ORPW suggests that the high ORPNR is related to low sleep pressure, while high ORPNR associated with low ORPW suggests a disorder that interferes with sleep progression (3).
Relevant References: