Americans' reticence to discuss a subject more taboo than sex is emphasized by the fact that 59 percent said they would rather discuss "the birds and the bees" with a stranger than their personal financial situation (39 percent).

Women in particular don't want to go there, at 63 percent, compared with 55 percent of men.

Among those 65 and older, 43 percent chalked up their nighttime worries to health care expenses.

Millennials, on the other hand, said their top three fears were job security, paying for housing, and being able to retire comfortably.

One reason people could be spending so many wakeful hours in the dark of night could have something to do with keeping up with the Joneses.

Thirty-nine percent of respondents said they felt under pressure to keep pace with friends and family members. But millennials really felt that peer pressure, with 59 percent of them feeling obliged to keep up appearances.

And that sort of casts into doubt another response: lying about one's finances.

Although 70 percent claim they haven't lied about their personal financial circumstances, the fact is that people are so hesitant to talk about money matters anyway, and in particular about being open to friends and family that they simply can't afford to maintain the same lifestyle, that it seems clear that either somebody's being less than truthful about money—or else that's why they're lying awake at night.

Those who did admit to embroidering the truth about their financial status most commonly did so about either their debts or their paychecks.

Forty-five percent didn't own up to the true amount they owed, while 39 percent didn't divulge the true level of their pay.