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Dynamic Sleep Rhythm Amplitude

SAFTE-FAST predicts Effectiveness as a function of time of day and prior sleep. The model was developed in the early 2000s using the science that was available at the time. The thing about science, though—the key thing, really—is that it changes. Our understanding of biological rhythms changes in response to new evidence. For example, the SAFTE-FAST team has been aware for some time now that the model's estimation of acclimation to a new time zone seems to take longer than what aircrew actually experience.

We know that performance is impaired by circadian misalignment, but importantly, circadian misalignment can occur in two major ways. The first cause of circadian misalignment is when we operate on a schedule counter-intuitive to the human propensity to be awake during the day and asleep at night. This type of circadian misalignment is seen in shift workers—individuals who are active at night need to sleep during the day. Daytime sleep has to fight two factors: 1) the urge to sleep (Sleep Intensity) is low because of signals from the body's internal clock, and 2) environment cues like light exposure, social activity, temperature, and noise all signal that it is time to be awake. These two factors impair sleep quality and make it more difficult to fully recover during a rest opportunity. The second type of circadian misalignment occurs when we travel to a new time zone. Now, the body clock is out of sync with environment cues for sleep. The local time and the environment is night, but our sleep intensity is low. It is harder to sleep because your body says, "be awake" but the local environment says, "go to sleep."

SAFTE-FAST models these signals through a feature called the Sleep Rhythm Amplitude Parameter. The strength of the Sleep Rhythm Amplitude is normally high during the body clock night and low at the body clock day, resulting in a strong urge for sleep that will be conducive to recovery. Our default Sleep Rhythm Amplitude has always been 0.5500 (an internal scale) and has been a good setting for shiftwork when sleep is during the day as it captures the combined effect of a poor "external environment" with the sunup and a poor "internal environment" – one's circadian function is aroused.

But in cases where multiple time zones are crossed, 0.5500 has been found to be too restrictive. It does not adequately capture the favorable external environment. When away from base at an out station and the external environment is favorable (sun is down, noise level is lower, and social distractions are less), in that case, only the internal environment's misalignment should be accounted for when penalizing sleep quality. In sum, previous versions of the Sleep Rhythm Amplitude prolong the period of "jet lag" more than it should have by not adequately considering the difference in external cues between day and night sleep.

The problem was that we previously penalized out-of-sync due to shift work and jet lag EQUALLY, even though sleeping in a new time zone does not involve the environmental disruptions of shift work. The solution is to modulate the amplitude of the sleep rhythm penalty based on how much of the sleep is occurring in the daytime. In short, SAFTE-FAST is switching from a Static Sleep Rhythm Amplitude to a Dynamic Sleep Rhythm Amplitude (DSRA) parameter. As shown in the figure below, the DSRA uses station data to dynamically change during day (0.5500), twilight (0.3300), and night (0.2200). Accounting for local day provides a much more realistic reservoir replenishment rate under the circumstances of transmeridian travel. When sleep is totally in the dark, the penalty is about 40% of what it is now. Ultimately, the solution is to modulate the amplitude of the sleep rhythm penalty based on how much of the sleep is occurring in the daytime. Using the same station data we already have that tracks light exposure throughout the year, the sleep rhythm amplitude settings will dynamically change during day (at 0.5500), twilight (at 0.3300), and night (at 0.2200). This way, the internal environment penalty for one's own body clock being out-of-sync will still happen. Still, the external penalty will fluctuate to account for how bad or good the external environment is for sleeping.

Figure 1.

The Dynamic Sleep Rhythm Amplitude functions by taking into account what percentage of sleep occurs during local night. At night, the penalty is lowest, followed by sleep during twilight. Sleep that occurs 100% during local day will have the greatest penalty.

As mentioned above, SAFTE-FAST modeling is based on the state of the science. So, let's discuss some of the science that informed the concept of the DSRA. A 2013 publication by Gander et al., entitled "Circadian adaptation of airline pilots during extended duration operations between the USA and Asia," looked at circadian adaptation among airline pilots before, during, and after trips where they flew from either Seattle or Los Angeles to Asia (7–9 time zones westward) [1]. Pilots spent 7–12 days in Asia and then flew back to the USA. While in Asia, sleep shifted by about 9 hours and was out of phase with the body clock, but there was no loss of total sleep time. Mean sleep efficiency declined across days in Asia, but the change was only about 3%, reflecting a relatively small impact of sleeping out of phase. This result suggests that the penalty for sleeping out of phase is relatively small when sleeping at local night, i.e., a dynamic sleep rhythm.

This minor change to SAFTE-FAST's underlying algorithm is a milestone event because it represents a fundamental improvement in the way we modulate the value of sleep to overall performance. This change was necessitated by applications of the model in situations involving multi-time zone shifts and driven by feedback from our users. Implementing the DSRA was previously delayed because there was little laboratory evidence to guide the change, but within the past 10 years, advancements to data collection technology and a global push towards field data collection has produced a number of sleep and performance studies conducted in a pilot population. We are also leveraging one of SAFTE-FAST's unique features-- its ability to track the location of the individual based on waypoints and estimate local daytime by location and date. Putting it all together, we were able to solve a problem in a way that other models have not yet addressed. This change is probably the biggest change to the SAFTE model in years. We are excited that this change, along with a few others, will become the default settings in SAFTE-FAST 7 for everyone. The Dynamic Sleep Rhythm Amplitude feature is currently available in SAFTE-FAST version 6.3 or later; however, it is not turned on by default. Contact your SAFTE-FAST Account Manager for more details.


1. Gander P, van den Berg M, Mulrine H, Signal L, Mangie J. Circadian adaptation of airline pilots during extended duration operations between the USA and Asia. Chronobiol Int. 2013; 30 (8): 963-972. Available from:


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