It could definitely be a perception thing or any difference with the situation in which you consume beer vs liquor. (IE, if you're drinking a lot of Jack and Coke maybe that caffeine is helping keep you alert?)
I think the most compelling answer is that there's actually a decent amount of research that shows hop compounds actually have a slightly sedative effect, especially in conjunction with alcohol. So...you know, beer.
National Institute of Health: The concentration of 2 mg of hop extract effectively decreased nocturnal activity in the circadian activity rhythm.
WebMD (Look under "Interactions"): Alcohol can cause sleepiness and drowsiness. Hops might also cause sleepiness and drowsiness. Taking large amounts of hops along with alcohol might cause too much sleepiness.