So many commuters jump on a Citi Bike in the morning that it's incredibly hard to find an open dock to park one in parts of Manhattan. In the afternoon, the problem reverses: too many workers are heading home, and all the available bikes disappear.

"Rebalancing" bikes to meet demand is a perennial challenge for bike-sharing programs, and Citi Bike has made extra efforts to serve riders in busy places such as Penn Station. But other neighborhoods are seeing problems, too — and it seems to be getting worse.

We analyzed data from July and August of 2015 to identify these Citi Bike "deserts," places where parking or checking out a bike is difficult during peak hours.  We found about 20 stations that are consistently full in the morning, concentrated in parts of Manhattan like Hudson Yards, the Flatiron District, and the west side of Midtown. If you head to these stations between 8 a.m. and 11 a.m., they're likely to show two or fewer open docks.

Last summer, you had a much better shot at finding an open dock at those same stations.

In the evening, good luck finding a bike anywhere in Midtown or the Wall Street/Battery Park area. About 40 stations in those areas stay consistently empty between 5 p.m. and 8 p.m., with one or zero bikes available.

If you look at an individual station, you can see the rhythms of the commute in action. For people who work near East 53rd St. and Madison Avenue, parking in the morning might be a challenge.  The station suddenly goes from nearly empty to nearly full around 8 a.m.

People who live near St. Marks Place and 1st Avenue have the opposite problem. The station has plenty of bikes early in the morning, but it empties out in a hurry around 8 a.m.

John Frost runs the bike share program for the New York City Department of Transportation. He says part of the problem is that the system is getting more popular.

“As people love it and jump on the bikes and ride more it gets harder and harder to keep the bikes and docks where they're needed when they're needed,” he said.

That problem keeps Citi Bike workers like Peter Meitzler very busy. Citi Bike employs people like him to rebalance the system by hauling bikes from full stations to empty ones throughout the day. Meitzler focuses on reallocating bikes in the Lower East Side.

“In the mornings, this neighborhood is off the hook,” Metzler said.