What goes up must come down — for proof, look no further than last week’s Broadway box office, which sank by more than $5.5 million in what was the first full frame of the Main Stem’s traditional post-holiday downturn.At musicals up and down the Rialto, and especially at the long-runners heavily reliant on tourist traffic, the declines were widespread and major. “Mary Poppins” (off 47% to $546,571) and “Chicago” (sliding 43% to $433,628), for instance, were among the hardest hit, while “Spider-Man: Turn Off the Dark” ($1,129,755) slumped by more than half a million dollars. On the silver-linings side, at least producers saw it coming, since B.O. goes into the same freefall every year after the Yuletide spike. And January, it turns out, can be a surprisingly good time to be a play. Almost every single nonmusical rose last week — including Lincoln Center Theater’s lauded revival of “Golden Boy” ($418,061), which logged its highest sales week so far. Scarlett Johansson starrer “Cat on a Hot Tin Roof” ($839,273) climbed 10% (following a prior sesh during which Johansson was out for a single show), and big-money Al Pacino topliner “Glengarry Glen Ross” ($944,347) returned to its usual seven-perf week following a frame of just five shows. The trend-busting uptick at plays in an overall slow week can be attributed to the fact that while razzle-dazzle tuners dominate the attention of holiday auds and family outings, the January slowdown allows local theatergoers — the primary demo for non-musicals — a chance to catch up on less flashy titles, often with the encouragement of wintertime ticket discounts. Hence similar rises at “The Heiress” ($444,970), starring newly anointed Golden Globe winner and Oscar nominee Jessica Chastain, as well as at “Who’s Afraid of Virginia Woolf?” ($281,137), “The Other Place” ($215,122) and “Picnic” ($193,935), despite the fact that both “Other Place” and “Picnic” opened last week. Tuners had to grin and bear it, but there was enough demand for popular offerings to keep four shows in the millionaires’ club. “The Book of Mormon” ($1,630,961) is making a habit of leading the pack in slow frames, last week topping the chart ahead of “The Lion King” ($1,620,752), “Wicked” ($1,519,043) and “Spider-Man” ($1,129,755). Broadway cume was down $5.8 million to $17.8 million for 24 shows on the boards — five fewer than the prior sesh, which included a spate of early-January closings. Attendance fell to 187,749, but that at least was close to 80% of overall capacity, which seems like not a bad tally for a winter frame. Week’s overall cume looks low compared to the $20.9 million logged during the same sesh last year, but that week included the weekend before the Martin Luther King Jr. holiday, which can often put a little wind in Broadway’s sails.