Law enforcement sources tell TMZ ... since January 6, the date of the violent insurrection at the U.S. Capitol, officers with both Capitol Police and the Metropolitan PD have been working a minimum of 12-hour shifts, 7 days a week, with no clear end in sight.

We're told a large number of cops are feeling drained with all of the overtime, and fear they won't be in good form come Inauguration Day -- when they need to be eagle-eyed.

Our sources say some Capitol Police staff are upset with leadership at the agency, because they feel they're most worried about image right now ... given the disaster on January 6.

Officers with the Metro PD have also been enlisted to protect the Capitol and buildings in D.C., and we're told they have many of the same grievances since the riots -- 12-14 hour days with no breaks ... and everyone's fatigued.

We're told Metro and USCP officers have been told they need to be on board with little or no time for family for a couple weeks ... possibly well into February if there's still a credible threat of unrest after the inauguration.

It has already taken its toll, with some law enforcement personnel resigning.

Our sources say a common question being asked by the officers is ... how are we supposed to be 100 percent if we're already spent?

The silver lining, according to our sources, is that officers are being allowed to take time off if they are having mental health struggles. We broke the story ... Capitol Police are being encouraged to seek counseling to deal with the deaths and trauma linked to the riots, and are getting the help they need.