"They would turn off the lights at 11:30, and I would just sit there and stare at the ceiling because I couldn't go to sleep," he said. "My heart was beating too fast. I would get night sweats. It was bad. I legitimately went through withdrawal."
Singleton desperately wanted to leave and wasn't open to the recovery process.
"But after I was there for so long, it just grew on me," he said. "I was like, 'I'm going to be here for 30 days, so I might as well get the best out of it that I can.' I used it as a learning experience."
At a time when he should have been getting in shape for spring training and a chance to make Houston's major league roster, he instead spent his days attending classes and therapy sessions with other addicts in a program for young adults.
One thing he didn't do: dwell on his missed opportunity.
"Not so much, because I knew I got myself into the situation so I had to deal with it," he said. "It wasn't like, 'I got myself here; now I hate myself.' It was like, 'I got myself here so I can't be mad at anybody but myself.'"
Although just 21 when he entered rehab, he already had a long history with marijuana, having used the drug "on and off" since he was 14. He blamed his start on the culture growing up in the Long Beach area in Southern California, where he estimated 80 percent of his friends knew not only where to get marijuana but how to get it within an hour.
Singleton clearly described his first experience with the drug and how it made him feel. To put it simply, he fell in love with that feeling.
And that's what drove his addiction.
"I guess I just don't like being sober," he said. "I like to change the way I feel."
In rehab, he had to identify why he needed marijuana and how to alter his behavior to avoid its traps.
"I've got to the point now where I know what I am," he said.
His stint in rehab helped him to quit using marijuana, and he said he hasn't smoked since -- a span of more than a year -- even though he was moved to Houston's 40-man roster in October and can no longer be tested for the drug.
Last season, when he made his debut in Triple-A after stopovers in low Class A and Double-A following his suspension, he struggled. He hit just .220 in 73 games, and his demons resurfaced.
"I went through some slight anxiety, some depression because I wasn't being successful," he said. "That was definitely difficult, and that drove me to drink."
He admits to abusing alcohol as a substitute for marijuana, getting drunk almost every day and "waking up hung over every morning."
After the season, he regrouped and prepared for the Puerto Rican winter league.
"I made up my mind to be my best, so hopefully better things happen because I'm not going out drinking and partying and doing all that kind of crazy stuff," he said.
The changes seemed to pay immediate dividends. He hit a league-leading nine homers in Puerto Rico and batted .268.
Singleton reported to Astros camp in good shape and in a better place mentally than last spring, hoping to show he is ready to compete for Houston's first-base job.
The team has been supportive during his battle, but Singleton knows he will have to stay clean to reach his goals.
He isn't receiving treatment for his addiction, isn't in a program and doesn't have someone traveling with him to keep him on track.