NEFL and bipolar disorder: Aggio et al.'s study [13] similarly found that NfL was mildly elevated in people with bipolar depression (n = 45) and linked this to neuronal distress with axonal damage due to an ageing‐related process, or “accelerated brain ageing.” Accelerated brain ageing refers to the age gap between chronological and the estimated neuroanatomical age, which has been observed in various psychiatric disorders including bipolar disorder [33, 34].