Table 3

Adjusted relative risk estimates for a traffic crash, for factors possibly related to drowsiness. Data from a case-control study in Washington State, 1997–98. Controls matched to cases on time of day, day of week, and location. Estimates adjusted for matching variables and age. Additional adjustments for confounders listed in table

Factors that may be related to drowsinessAdjusted relative risk of a crash95% confidence intervalAdditional confounding variables
*95% confidence interval excludes 1.0.
Trip miles driven by driver (100 mile units)2.21.4 to 3.3*Hours since trip started
Hours since trip started (2 hour units)0.70.5 to 0.9*Miles driven by driver
Hours since last sleep
    <61.0Reference categoryRoad surface (dry, wet, snowy)
    6–110.90.3 to 2.3
    12–170.20.0 to 0.7*
    ≥181.30.4 to 4.0
Average hours slept per day in last week1.10.9 to 1.3None
Average sleep hours needed per day1.00.8 to 1.3None
Breaks taken on trip
    Any compared with none1.20.7 to 1.9Number of breaks beyond first
    Number of breaks beyond first0.90.7 to 1.0Any break compared with none
Used highway rest stop0.50.3 to 1.0*Miles driven by driver
Sensation of falling asleep on trip14.21.4 to 147*Time awake in last 24 hours
Yawned while driving0.40.2 to 0.7*None
Time since last ate (6 hour units)0.80.6 to 1.1None
Radio, tape, or compact disk player on0.60.4 to 1.0*None
Window open1.10.7 to 1.8None
In last 2 hours, drank
    Coffee0.50.3 to 0.9*None
    Tea0.40.1 to 1.6None
    Soda with caffeine1.71.0 to 2.8None
    Any caffeinated beverage0.80.5 to 1.3None
Past crash due to falling asleep5.91.0 to 36Miles driven by driver
Hours since trip started
Road surface (dry, wet, snowy)
Ever fell asleep at the wheel1.61.0 to 2.7Miles driven by driver
Hours since trip started
History of sleep apnea diagnosis2.90.2 to 45Road surface (dry, wet)
Snore most nights1.70.8 to 3.4Road surface (dry, wet, snowy)
Gasp in sleep1.00.5 to 1.9None
Epworth sleep score (in units of 6 points)1.60.9 to 2.8Road surface (dry, wet, snowy)
Miles driven by driver
Hours since trip started