Ex-NFL star Robert Mathis told police he drank alcohol and took a sleeping pill before getting behind the wheel of a pickup truck ... this according to the police report obtained by TMZ Sports.

Mathis was pulled over after cops noticed him driving the wrong way down a 1-way street early Tuesday morning -- and when officers approached the vehicle, Mathis was a mess.

The officer says, "As I asked Robert Mathis for his driver's license and vehicle registration, I noticed that his speech was slow and thick and he appeared lost and confused."

When asked for vehicle registration, Mathis handed the officer a large vehicle placard (a placard that comes with brand new vehicles).

According to the report, officers asked Mathis if he had anything to drink -- "He stated that he had one shot of cognac, but had also taken a sleeping aid around 10 PM."

Mathis told another officer he had two shots of Crown Royal along with the sleeping aid.