Do Western teams dominate in NFL night games?

You're getting ready for an NFL night game.  A team from the Eastern time zone is playing a team from the Pacific time zone.  How should you bet?

From 1970 to 2011, you should have bet on the West Coast team.  In games starting at 8:00 pm (Eastern) or later, they beat the spread 70 out of 106 times.  That's a record of 70-36, or .660.  The odds of something that extreme happening by chance (either way) is 1 in 806.  It's 3.3 standard deviations from the mean.

In a control sample of afternoon games, there was no such effect.  In fact, the Western teams went only 143-150 (against the spread) in those.

What's going on?  Well, the academic authors who found this result claim it's due to the circadian rhythms of the human body.  Physiologists and psychologists believe athletic performance peaks in the late afternoon.  So, for a game that starts at 8:00 pm Eastern time, the players from the West are actually playing at 5:00 pm "body time," which is why they perform better.

That result comes from a recent academic study: "The Impact of Circadian Misalignment on Athletic Performance in Professional Football Players," by Roger S. Smith, Bradley Efron, Cheri D. Mah, and Atul Malhotra.  Here's a Business Week rundown that actually just came out today.  Deadspin mentioned it here, and Brian Burke mentioned it here.

When I read the reports, I couldn't believe that the 70-36 could actually be accurate.  I downloaded the study ($8), and then went to Pro Football Reference to confirm for myself with their game finder.  And, yup, it all checks out!  

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Wow, eh?  Could that actually be what's happening, that time-of-day effects on the human body are so big that they're almost twice the size of the home field advantage?  

Well, as you might expect, I'm skeptical.  I can think of a whole bunch of other things that might be going on.

Nothing of what I'm going to say is conclusive ... you should take this post not as a definitive rebuttal, but, perhaps, as a case for the defense, a "devil's advocate" kind of argument.

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1.  There's no actual evidence that the West teams played better.  The only data we have is that they consistently beat the spread.  

It's just as possible, isn't it, that the bookmakers were shading the spread in favor of the east teams, at least in the night games?

I broke down the night games by day of week.  (They don't quite add up because I did everything manually, and probably screwed up somewhere.)

Sunday: 18-12
Monday: 46-19
Other:   3- 7

It turns out almost all the effect happens on Monday.

Does this support the "line shading" hypothesis?  I think it does, a little bit.  Monday night games, I'd imagine, get the most action from bettors ... if bookmakers shade the line when betting gets too heavy and unbalanced, it seems like Monday games should be the best candidates for when that happens.

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2.  Night games are not random.  They're selected specifically because the NFL believes that they'll be the best games.  Maybe they're what the NFL thinks will be the games with the best teams, or the most exciting games, or the games with the most serious playoff implications.

For the most part, the NFL schedules the night games before the season starts (the exception: late-season Sunday games, which, in recent years, are chosen on the fly).  So, the league is, to some extent, guessing which the good teams are.  

Because of that, certain teams will appear on Monday nights more than others.  In this particular sample, the San Francisco 49ers appeared 28 different Mondays; the Seattle Seahawks, only six.

Could it be that there's something about the 49ers that caused them to beat the spread so much?  Perhaps the oddsmakers, and bettors, consistently underestimated San Francisco, those years.  For 16 consecutive years -- 1983 to 1998 -- the 49ers went 10-6 or better.  You'd think they'd have regressed to the mean at some point, but they didn't.  So, perhaps they were consistently lucky?

There's a bit of support for that -- every year from 1983 to 1989, the Niners had a winning record against the spread. Over their entire streak, they never went worse than 7-9.  Of course, a lot of that is due to their 19-9 record on Monday night ... but, still.

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3.  Suppose that we assume that SOME of the effect is due to these kinds of factors.  Suppose that, because of shaded or incorrect spreads, the Western teams had a 53% chance of beating the spread, instead of 50%.

In that case, the odds of a 70-36 record now drop to only 1 in 225.  That gets a bit easier to accept as random chance.  

At 55 percent, you're down to 1 in 73.  

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4.  If an effect actually exists, it's most likely a lot lower than what was actually observed.  

First, you need to regress the 70-36 to the mean, since, in nature, small effects are found much more frequently than large ones.  

Second, any effect smaller than 2 SD wouldn't have been published, which means, in general, effects that DO get published are overestimates.  

The study found the western teams beat the spread by an average of 5.26 points, with an SD of 1.33.  That means any result less than 2.66 points wouldn't have made it into print.  

Suppose that, unbeknownst to us, the actual circadian effect is 2.5 points.  If every study takes a different random sample of 106 games, fewer than half the studies will find statistical significance.  And ALL of those studies that *do* find significance will overestimate the real effect, because the minimum significant effect is 2.66.

That wouldn't be a problem if the SD was, say, 0.01, or something, because then almost ANY real effect would be found.  But, in this case, when the bar is set so high, the selectively-sampled observed effects are likely to be inflated by luck.  

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5.  The effect for home games and road games is almost the same.  That is, no matter which of the teams is jet-lagged, the West team has the same advantage.  

That's fine, if the theory that only time of day matters.  

It does imply, though, that jet lag, or adjusting to a new time zone, doesn't matter much at all.  Which may be true, but I've seen other psychologists argue the opposite.  

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6.  If there's such a huge effect for a three-hour difference, you'd expect to still have a substantial effect for a two-hour difference.  So I checked west-coast teams playing night games on the road in Central Time.  In those games, the effect disappeared.  The Western teams went 17-26 against the spread.

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7.  If the effect does depend on time of day, then the effect should be similar for the fourth quarter of late-afternoon games, right?  Those games might end at 7:00 pm, while the night games start at 8:00 pm.  Not much difference there.  

Does that happen?  I haven't checked, but that would be a good test.  You could also see if the effect diminishes as the night game goes on ... by the end of the Monday night game, the West team is playing at 8:00 or 9:00 pm circadian time.  Of course, the East team is at the actual time of 11:00 pm to midnight, but, as far as I read, the paper doesn't posit that there should be a big difference between early evening and late evening.

(UPDATE: One author says the effect is based on distance from the 3:00 am physiological low point.  Going with that ... an 8:00 game is 10 hours benefit for the PST team, and only 7 hours benefit for the EST team.  But, then, an afternoon 1:00 game would be the opposite, 7 hours to 10!)

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8.  You could also check late-afternoon games in general.  The East team is on 4:00 time, while the West team is on 1:00 time.  So East should have the advantage.  But, the authors explicitly say (in the Business Week article) that the "ramp up" effect is smaller than the "ramp down" effect, so maybe you wouldn't see anything.  

And, while we're here ... Daylight Savings Time.  The effect should be different the week the clocks change, right?  Instead of the West team playing at 5:00 and the East playing at 8:00, it's really (from a circadian standpoint) 6:00 and 9:00.  

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9.  The author published a similar study in 1997, with similar results.  But the effect continued -- which suggests that bettors didn't react to it, and bookmakers didn't bother adjusting their lines.  

It's possible that the betting community was just shortsighted in not believing the paper's claims, but ... sharp bettors are usually quick to seize on inefficiencies like this.  To me, that's at least a bit of evidence that it might be something else.

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10.  There are afternoon games in many other sports -- college football, NBA, NHL, major-league baseball.  You could check to see if this happens there.  

Even better, you could check sports in which *actual* performance can be measured, not just performance relative to another team.  Do golfers hit better in late-afternoon?  Do Rubik's-cube solvers have better times in events that take place later in the day?  What about bowlers, or dart-throwers?  There should be lots of ways to check.  

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11.  In fairness, My view is that someone would have noticed if there were large time-of-day effects in other sports.  

From my own introspection, I think I'm better certain times of day than others.  I tend to get drowsy late in the afternoon, and I wouldn't be surprised at all if my (extremely minimal) athletic ability drops during that time, and other times I'm tired.  

But, I *notice* my tiredness.  Shouldn't professional athletes have noticed something, too, especially when they're so focused on their bodies and their performances?

Maybe it's possible for a team to drop from .500 to .333 without noticing the physiological changes that caused it, just like they may not notice any difference when they play worse on the road.  But ... I dunno, my gut says that's just too big an effect that nobody even *suggested* it before the academics.

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12.  Oops!  As I was doing the final edit for this post, I remembered an NBA study that found Western teams travelling east had an advantage.  I checked, and the advantage was huge, just like this one.  And most NBA games are at night.  So ... hmmm.  

However, that study wasn't as clean as this one, with a complicated regression.  And it was denominated in actual winning percentage, rather than against the spread.  But, still ... hmmm.

In fairness, I have to say that study supports the pro-circadian argument, to some extent.

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My completely arbitrary, intuitive, Bayesian guess as to what's actually causing this effect?  I'd say ... 20 percent line shading, 75 percent luck, and maybe 5 percent physiology.  

I'd also guess -- again, without any justification other than my gut -- that there's a 35 percent chance that there's no real measurable effect of circadian physiology at all, and a 65 percent chance that there's a measurable, but small, effect.  (I had it at 50/50 before I recalled the study in #12.)

Regardless, I definitely don't want to imply that this study isn't important.  Any time you find a huge, 70-36 result, after a prior prediction with a plausible mechanism ... well, that's something you definitely want to put out there for serious consideration.  I'm just not as confident as the authors that what they've found is an actual thing.

Prove me wrong, somebody!