Inclusion Criteria:  ≥18 years old US military veteran (not necessarily military) Currently has PTSD diagnosis as determined by clinical diagnosing Screening score on CAPS minimum of 30 (per discussions with Dr. Murray Raskind and data from previous studies 32) Has score ≥3 on CAPS nightmare items B2 and E6 Speaks and understand English Willing to wear an actigraph and come into the clinic as programmed  Exclusion Criteria:  At Moderate or High risk of suicide based on results from the Columbia-Suicide Severity Rating Scale (CSSR-S) screen version - recent. Acute or unstable medical illness Has acute or unstable mental illness or any cognitive issues which the PI determines would interfere with engagement in the study (e.g., active schizophrenia, uncontrolled bipolar, history of neurocognitive impairment, history of moderate-severe traumatic brain injury) Currently receiving exposure therapy Recently enrolled (<1 month) in other behavioral health therapies (exclusions made at the PI's discretion depending on therapy type and length since admission) Were prescribed clonidine within the last 6 months Blood pressure under 90/50 or symptoms of low blood pressure (light headedness, dizziness, heart palpitations, or other symptoms as determined by clinician).  Any contraindications of taking clonidine such as:  Known hypersensitivity to clonidine History of 2nd or 3rd atrioventricular block History of sinus bradycardia History of pheochromocytoma History of Raynaud's phenomenon Stage 5 Kidney disease Recent myocardial infarction (<6 months) History of cerebrovascular disease or recent stoke (<6 months)  Currently have any of the following diagnoses:  Opioid use disorder Cocaine use disorder Alcohol use disorder Cannabis use disorder Sleep apnea diagnosis with verbal indication of non-adherence to treatment  Current prescriptions for:  Prazosin or another α1-adrenergic antagonist Any opiate (e.g., buprenorphine, hydrocodone, oxycodone) Antipsychotic medications Benzodiazepines Cyproheptadine Precluded from taking clonidine for medical or other reasons Based on PI assessment is cognitively unable to engage in the study