Becoming Batman

by E. Paul Zehr

  The main function of the body clock is to ensure standard cyclical regulation of hormonal levels in multiple systems in the body. In humans, the body clock is also affected by information about physical activity and overall level of stress or excitement. The key point is that the internal clock is synchronized with the external world by cues taken from light levels and physical movement related to activity and exercise. This makes sense if you consider that low light levels at night would naturally coincide with low levels of physical movement. In contrast, the opposite would be expected during the day. The relation between light, additional melatonin taken from outside the body (in a pill for example), and exercise and the body clock is shown in Figure 12.1.

  Figure 12.1. The body clock is affected by exercise, levels of light, and melatonin concentration. The normal release of melatonin from the pineal gland is triggered by decreased or dim light levels at night (internal melatonin). The highlighted section at right shows minimum body temperature, which is the anchor of the circadian cycle. Modified from Waterhouse et al. (2007).

  An important reference point is the rise and fall of body temperature mentioned before. The circadian rhythm is anchored to the minimum core temperature observed during the day, which typically occurs some hours before waking. Bright light shifts the body clock backward when it is provided before the time of minimum core temperature and forward when given after the time of minimum core temperature. That is, light makes you less sleepy in both cases. Exercising early in the evening moves the clock forward, but exercising at night, during what would be the early part of normal sleep, shifts the clock backward. So, light and exercise can push the body clock in different directions based on when they are enhanced relative to the normal sleeping cycle, particular in relation to the normal light-related release of melatonin and body temperature minimum. These points essentially anchor the beginning and late phases of sleep.

  Every day there is adjustment of the internal body clock to the external environment. These adjustments are mostly from light-related cues called “zeitgebers” (meaning “time givers” in German). When we are living and working in a standard relationship to the daily light levels in our region, the body clock stays nicely synchronized and our mood and energy levels fluctuate in a consistent cyclical pattern. While you probably haven’t strongly considered the link to rising body temperature, you have certainly noticed that your mental performance likely improves in the hours after you wake up. This isn’t just because you have had your morning coffee, by the way! This trend generally continues on into the early evening. After that point, deterioration in mental productivity occurs.

  Unless Batman has a way to reset and reanchor his body clock (which we will explicitly discuss in a few pages), this effectively means that he will be called on to do his best—and most dangerous—work when physiologically he is on the falling edge of performance! Many aspects of physical activity, including peak muscle forces, anaerobic power output, and motivation to perform or sustain activity, rise and fall on circadian rhythms. Even something as simple as spinal reflexes changes according to circadian rhythms. Shown in Figure 12.2 are reflexes (similar to the stretch reflex discussed back in Chapter 7) taken every four hours across a 12-hour period from 8:00 a.m. to 8 p.m. The important point is that the size of the reflex increases steadily from early morning until early evening.

  Figure 12.2. Change in nervous system excitability in the spinal cord over the course of a day. Relative time points for Batman’s daily activities are shown for comparison. The black arrow indicates the progression of time during a single day. Modified from Lagerquist et al. (2006).

  The levels of many hormones in the blood also follow a daily pattern. Cortisol levels in blood plasma are lowest just after noon and reach their peak just around late evening. These rhythms are partially synchronized to our daily routine. But at the base level our rhythms across the day—circadian rhythms—are tied to the level of light we are exposed to during the day and at night.

  If you look at Figure 12.3, you will see the levels of melatonin (A), body temperature (B), subjective alertness (C), and task performance (D) across a 24-hour period. Also shown is the level of triacylglycerol (E), which is used as a marker for metabolism. In this figure the dashed vertical line indicates the time of minimum core body temperature that we discussed up above. The key thing to notice is the close parallel of these levels as they change across the day.

  Figure 12.3. Levels of melatonin (A), body temperature (B), subjective alertness (C), and task performance (D) across a 24-hour period. Also shown is the level of triacylglycerol (E), which is used as a marker for metabolism. The dashed vertical line indicates the time of minimum core body temperature (see Figure 12.1). Courtesy Rajaratnam and Arendt (2001).

  When the Bat Hitches a Ride on a Jet Plane

  There is evidence that working night shifts can throw off our diurnal patterns of metabolism and cognition and may be unhealthy. What are the effects Batman should be concerned with for working his night shifts? For that matter, we can easily imagine similar factors at play for firefighters, police officers, or ambulance workers. However, before exploring the extremes of shift work, which involve an attempt at chronic and long-term change in circadian rhythms, I want to begin first with shorter term and brief changes in rhythm that just about everyone of us—shift workers or not—have experienced at one time or another. What I am referring to are the effects of seasonal changes in light levels and the effects of air travel and what is known as “jet lag.”

  Jet lag is really a minor, temporary version of the bigger issues that arise with chronic night shift work. The basic concept to carry forward from our discussions above is that we have a certain set point in our circadian rhythms and our sleep-wake cycles. These points are not set in stone, so to speak, but are subject to seasonal variations. These changes across the seasons vary depending on where we live. They are gradual and minor at latitudes near the equator and extreme near the poles. In Anchorage, Alaska, for example, the longest day of the year in 2007 was June 21, when sunrise was at 3:30 a.m. and sunset at 10:42 p.m. This extreme “light” day can be contrasted with the winter solstice of December 21, 2007, when sunrise was at 10:14 a.m. and sunset at 3:41 p.m. Then contrast both of these with Quito, Ecuador, where the sunrise and sunset times for the summer solstice were 6:12 a.m. and 6:19 p.m. and for the winter solstice 6:08 a.m. and 6:16 p.m. Across the seasons, then, the ambient light levels fluctuate from a daily range of about 20 hours in the summer to about four and a half hours in the winter in Anchorage and pretty much evenly 12 hours for Quito.

  However, even if we live at a latitude like that of Anchorage where there are extreme differences in the actual hours of daylight between summer and winter, we don’t move in one 24-hour period from summer light levels to winter ones. Instead, the changes across the seasons are typically very gradual. Because of that, our set point can slowly adjust—mostly—to these differences. Despite that, it can still be very difficult, and there can be considerable differences in an individual’s ability to adapt to the seasonal differences in the extreme north and south. As mentioned above, a key regulator of sleep and waking cycles is melatonin. There seem to be two points of melatonin secretions across a normal day, and the off and on signal of the secretions help trigger the sleeping and waking cycles. The melatonin cycles themselves are triggered by levels of ambient light. This is why there can be such large differences in set points across the seasons.

  However, very large and abrupt changes happen when we travel from one part of the world to a completely different part with a radically different time zone within a single day. This is now quite common because of ease of worldwide travel via airplane. This change in our physical location means our set point is no longer in line with the actual ambient light levels in our new time zone. We may experience dramatic effects in blood pressure, body temperature, and hormone release. Over time, though, we do adapt to the different time zone. Then we get to repeat the process when
we return home!

  A Serious Example

  Let’s pause here to consider an interesting example of where jet lag can have very widespread and significant impact in important activities related to physical activity. This is going to be a serious pause for a very important issue so please pay attention. I am speaking, of course, about Monday Night Football and the National Football League. If you think carefully about all our discussions about circadian rhythms and in consideration of your own experience being the owner of your own human body with varied alertness and energy across a day (and you can look at Figure 12.3 for the physiology behind this), you might begin to wonder what would be the effect on your overall performance of switching time zones to the same local time in different parts of the country. An excellent example of this is professional sports events and in particular those staged in North America where regular daily travel across three or more time zones occurs to play football, basketball, hockey, or baseball. Given all of this, it might be supposed that competing at certain times of the day might offer an advantage for some teams traveling out of their time zone.

  This was precisely what Roger Smith and colleagues at Stanford University wondered with regard to NFL teams playing on Monday Night Football. They examined the outcomes of 25 years’ worth of Monday Night Football games when teams based on the West Coast played teams based in the East. This gave them 63 games to consider. This is an interesting comparison because these Monday night games are always played at 9:00 eastern time, regardless of where the game will be held and from where the teams are actually traveling. Smith and colleagues thought that West Coast teams should have an advantage since the actual game time would always be earlier in circadian body clock time. Hold on, you might say at this point, this whole idea would only work if you had some way to control for the fact that different teams from the East and West are always playing and those different teams are going to be of different skill and caliber. That is going to mess up any comparison related to circadian issues, which should be smaller anyway. However, these scientists where actually quite clever and turned to other experts to give information about who should have done better or won each game. The other experts are, of course, the ones who set the Las Vegas point spread.

  The results of the analysis showed that West Coast teams won more often and by more points per game than did East Coast teams. In fact the winning percentage for West Coast teams on Monday Night Football was 63.5% compared with only 36.5% for East Coast teams!

  You might wonder if maybe the East Coast teams just didn’t play well under the pressure of Monday Night Football. After all, there would be more viewers actually in the East watching the games. Maybe that entered into the players’ heads during the game and affected performance somehow. However, when those East Coast teams with the 36.5% winning percentage played other East Coast teams, they won 65.2% of the time. The scientists suggest that this is an example of getting increased performance by not adjusting the circadian clock to the new time zone and therefore gaining a performance benefit. This can be considered in the context of a “one off” event but probably doesn’t have much carry forward to ongoing performance as the Dark Knight.

  A lot of work has gone into attempts to develop schedules to minimize the effects of jet lag. These are largely based on using the key contributors of the circadian rhythm—light and associated melatonin levels. From these, elaborate schedules involving progressively shifting the ambient light levels in advance of the flight have been suggested. For example, Charmane Eastman of Rush University Medical Center in Chicago has come up with an idea (shown in Figure 12.4) that involves using bright light to help shift the circadian rhythms before a flight.

  Figure 12.4. Schedule for in altering circadian rhythms before a flight to reduce jet lag in travel from North America to Europe. The four main North American time zones and one for Europe (Paris) are shown at top. The sleep schedules at bottom are from seven days before the flight (Day –7) to two days after the flight. Internal melatonin secretions triggered by dim light levels are shown by the arrow. The minimum body temperature is indicated by the open triangles. Ls mean the administration of intermittent bright light for two to three hours each morning. Adopted from Eastman et al. (2005).

  The schedule shown in the figure is for travel from North America to Europe. Here the top time axis shows the four main North American time zones and one time zone for Europe (Paris). The sleep schedules are shown from seven days before the flight (Day –7) to two days after the flight, moving the body clock forward by one hour per day. That means that the time for awakening is advanced one hour per day as well.

  If Batman were flying from Gotham City (eastern time zone) instead of from Los Angeles, fewer days in the schedule would be needed. Anyway, using this example from Los Angeles, Batman / Bruce Wayne would arrive in Paris already adjusted to the new time zone. It is possible, then, to deliberately modify the body clock as long as the important timing cues—zeitgebers like bright light exposure—are used.

  However, jet lag, for most people, is a rare thing. Now let’s look at how much our—or Batman’s—body clock set points can be altered long term for activities such as shift work.

  Working the Batshift

  Some of the considerations we already described for travel and jet lag also apply for shift work. Probably it is best to consider jet lag as an intermittent and temporary change to the body clock set point. Regular shift work can be ongoing and permanent—always working the same night shift—or ongoing and rotational, as with police officers whose shifts rotate around through days and nights.

  Whether permanent or rotational, when we start thinking about night shift work, we have moved into the territory of a permanent (while we work that job at least) change in a set point. However, the set point changes are still heavily dominated by light levels. Because of that, we are usually working against the body for the circadian rhythms to be synchronized to the light levels. A big problem for night shift work is that it is unlikely that the internal body clock is permanently shifted to the requirements for the work environment. This is opposite to the situation with jet lag, then, but central to what Batman does (and notably emergency personnel such as firefighters, police officers, and ambulance workers do) on a daily basis.

  There are some documented hazards associated with chronic night shift work. Eating meals at unusual times relative to the internal circadian rhythms can increase the risk for cardiovascular disease, hasten the onset of diabetes mellitus, and lead to ulcers. Also, since Batman is sleeping mostly during the heart of the day, he is not out in the sun. Being out in the sun is where we get loads of vitamin D, which is essential for bone metabolism. By the way, vitamin D doesn’t directly make your bones stronger. What it does is dramatically increase the absorption of dietary calcium (by between about 30 and 80%). So, it is very important for bone health. Vitamin D3 is produced in your skin by the pigment melanin (not melatonin, although the words are similar) when the skin is exposed to ultraviolet light. So sporting around at night (when ultraviolet light levels are virtually nonexistent) or while wearing a batsuit means that Batman gets very little vitamin D from sunlight exposure. Batman needs to drink lots of fortified milk or use a tanning bed!

  Batman is forced to do his patrols and deal with the crazy criminal element of Gotham when his body is physiologically ready for sleep. Then he has to come home late (early in the morning) and try to sleep when his body is trying to get ready to be awake. Because of that, any sleep that Batman will get after his night shift is likely to be about two hours shorter than what he would get if he were sleeping at night. This also highlights another problem. Batman and most other shift workers go to sleep after their night shift, not before. If you think for a moment you will see that this is the reversal of a normal daytime schedule where you wake up and go to work.

  We already talked about why having a misaligned body clock was a problem for mental alertness and physiological function when we were thinking about jet la
g. What can be done to help out in the case of shift work? As with jet lag, the dominant factor seems to be levels of ambient light. The solution, then, is actually quite simple but is very difficult to implement. Basically, keeping well adapted to the new requirements of a nocturnal work schedule means trying to maintain a new light-dark cycle. That means reducing light levels when they should be low but are actually naturally high (during the day) and increasing light levels when they need to high but are actually low (at night).

  This all seems simple enough. However, the body clock is extremely sensitive to light and any inappropriate bright light exposure can derail the best attempts to shift the body clock to a nighttime phase. It is a bit like checking on how a cake is baking or bread is rising. The very act of checking it can make it fall! Well, letting in even a little light at the wrong time can do the same thing for circadian rhythms. Things that have been tried include having a night shift work environment as bright as possible (to increase light in the new “day”) and wearing very dark glasses to go home after the shift and heavily shrouded curtains in the room where daytime sleep will occur (to reduce the light exposure in the new “night”). However, the necessary sunglasses can actually be so dark that they are dangerous to use while driving!

  This wouldn’t be that much of a problem for Batman, since he does have a chauffeur and butler in Alfred, and who can just go to bed when he returns from patrol. However, this is a real and significant issue for the example night workers of emergency personnel. Also, because these workers often do other things (like take kids to school) when they get off the night shift they wind up with all that additional ambient light at the wrong time for their body clocks and are therefore constantly reshifting their body clocks. For Batman there are a couple of problems with this. He is out in the dark swinging around Gotham City. He cannot really do his job while using a giant high-intensity light! It is worth noting that the permanent mismatch between biological body clock and environmental zeitgebers virtually guarantees that Batman never adapts fully to his night shifts. He will be tired almost all the time.