Do you need a nap? um... test with this ONE WEIRD TRICK! :D

[Staggerwing]

I'm more interested in the 'too tired to drive' tie-in. What's their explanation? Tiredness leads to slight nystagmus?

[JasonPratt]

I've been in situations where my mind and thus eyes were so fatigued that, at night, my eyes were dangerously wandering all over the windshield and dashboard. The optical illusion forces eye-drift, which tests how well we're able to focus attention at the moment.