Lindsay Lohan's gonna have a hard time staying clean behind bars -- TMZ has learned she will only be allowed to shower every other day once she checks into Lynwood Correctional Facility tomorrow.
Law enforcement sources tell us the jail is all prepared for Lindsay's arrival -- where she'll be staying in her own cell ... in a special module where solitary inmates are held.
We're told Lindsay will not be allowed to interact with other inmates while she's inside her cell -- which could be as much as 20 hours per day. She will be allowed to exercise three times per week.
As far as her diet, we're told Lindsay will get two cold meals and one hot meal every day. Today, inmates had chicken stir fry for lunch ... so it's not too bad.
We're told there's no guarantee that Lindsay will be allowed to take all of the medication she's been prescribed -- and jail officials will determine what she can take after she arrives.