What it isA nexus letter is a written opinion from a licensed physician or healthcare provider stating, at minimum, that it is ‘at least as likely as not’ that a veteran’s current medical condition is related to their military service. It is the primary tool for establishing the medical nexus required for VA disability service connection.

Why it mattersWithout a nexus letter, most VA disability claims for sleep disorders — including insomnia, OSA, and nightmare disorder — will fail on the nexus element. A strong nexus letter from a sleep medicine specialist who has reviewed the veteran’s service records is the single most effective way to strengthen a claim.

Think of it like thisA nexus letter is the bridge between a veteran’s service and their diagnosis. The VA needs someone with medical credentials to stand at both ends of that bridge and confirm it is solid. The nexus letter is that person’s formal statement.

Under 38 CFR § 3.303 and related regulations, service connection requires a three-part showing: current diagnosis, in-service incurrence or aggravation, and a causal nexus between them. The nexus letter provides the third element through a competent medical opinion meeting the ‘at least as likely as not’ (50% probability) threshold required by VA adjudication standards.