Lindsay Lohan -- Bike Curious

Lindsay Lohan is telling friends ... she's committing herself to a healthier, more active lifestyle ... cutting way back on the late-night partying -- and what better way to do that ... than to BUY A BICYCLE.

Sources close to Lindsay tell TMZ, the actress is beginning to face facts -- it's NOT a good idea for her to party at all hours of the morning inside bars, hotels, and restaurants.

Not only is it a terrible influence on a recovering addict -- Lindsay is saying ... it gives people the impression that she's back to her old ways ... when she's not.