The 30-year-old star said she took the prescription pills for four years and only kicked the habit after three weeks in rehab in South Africa.
She said: “I’m just glad I survived.
“My drinking was out of control and I was addicted to sleeping pills. It was an awful combination.”
The chart star – who scored hits such as Love Machine and Biology – with the girl group sought help for depression and alcohol abuse last October when her relationship with DJ Crane fell apart.
The singer, who said her bandmates had been in touch to support her, plans to concentrate on her acting and music career.