Lindsay Lohan's Scary Time In Jail

We're told Lindsay was alone in her cell during her time in custody at Century Regional Detention Facility in Lynwood, CA.

Sources tell us Lindsay described the jail as "cold" -- saying it's one of the worst places she's ever been in her life.

Lindsay is telling friends the experience was another wake up call ... insisting she's learned from her mistakes and wants to put her problems behind her.