In the two hours around bedtime, TV viewing was the most common activity. In fact, watching television accounted for almost 50 percent of the activities undertaken in the time before bed.
The study will be presented Monday at the Associated Professional Sleep Societies annual meeting, in Seattle.
The finding suggests that television, rather than hours past sunset or biological signs, has become the most important signal for sleep.
Researchers say staying up to catch the end of a favorite show may make people stay up later than they otherwise would.
"These aspects of modern life are potentially reducing sleep time below what is physiologically required", the researchers noted in a news release from the American Academy of Sleep Medicine.
Researchers note that getting less than seven to eight hours of sleep daily can lead to impaired alertness and has been linked to higher rates of obesity, illness and death.
Americans should watch less late-night TV and go to work later in the morning, the researchers suggested.