Inclusion Criteria:  Male or female participants must be aged 16 years or older at the time of signing the informed consent/assent. In participants aged <18 years, height velocity must be less than 2 cm/year in the last year and puberty must be completed (Tanner stage V). Participants with known classic CAH due to 21 hydroxylase deficiency diagnosed in childhood with documented (at any time) elevated 17-OHP and with or without elevated A4 and currently treated with hydrocortisone, prednisone, prednisolone or dexamethasone (or a combination of the aforementioned glucocorticoids) and on stable glucocorticoid therapy for a minimum of 3 months. Participants who are receiving fludrocortisone must be on a documented stable dose for a minimum of 3 months prior to enrollment and must have stable renin levels at baseline. Female participants of childbearing potential and all male participants must agree to the use of an accepted method of contraception during the study. A female participant is eligible to participate if she is not pregnant, not breastfeeding, and she is either not a woman of childbearing potential (WOCBP) or has a negative pregnancy test at entry into the study. Note: females presenting with oligomenorrhea or amenorrhea who are aged ≤55 years should be considered potentially fertile and therefore should undergo pregnancy testing like all other female participants. Capable of giving signed informed consent/assent which includes compliance with requirements and restrictions listed in the informed consent form (ICF) and in this protocol.  Exclusion Criteria:  Clinical or biochemical evidence of hepatic or renal disease e.g. creatinine >2 times the upper limit of normal (ULN) or elevated liver function tests (alanine aminotransferase [ALT] or aspartate aminotransferase [AST] >2 times the ULN). History of bilateral adrenalectomy. History of malignancy (other than basal cell carcinoma successfully treated >26 weeks prior to entry into the study). Participants who have type 1 diabetes or receive regular insulin. Persistent signs of adrenal insufficiency or the participant does not tolerate treatment at the end of the 4-week run-in period. Participants with any other significant medical or psychiatric conditions that in the opinion of the Investigator would preclude participation in the study. Participants on regular daily inhaled, topical, nasal or oral steroids for any indication other than CAH. Co-morbid condition requiring daily administration of a medication or consumption of any material that interferes with the metabolism of glucocorticoids. Participants who are receiving <10 mg hydrocortisone dose at baseline or the hydrocortisone dose equivalent. Participants anticipating regular prophylactic use of additional steroids e.g. for strenuous exercise. Participation in another clinical study of an investigational or licensed drug or device within the 12 weeks prior to screening. Inclusion in any natural history or translational research study that would require evaluation of androgen levels during the study period outside of this protocol's assessments. Participants who have previously been exposed to Chronocort in any Diurnal study. Participants who routinely work night shifts and so do not sleep during the usual night-time hours. Participants, who in the opinion of the Investigator, will be unable to comply with the requirements of the protocol.