Why Carpal Tunnel Strikes at Night: The Real Anatomical Reason
Carpal tunnel syndrome (CTS) is classically described as a nocturnal condition. Many patients report waking with numbness, tingling, or burning in the thumb, index, middle, and radial half of the ring finger. These symptoms often improve temporarily by shaking the hand. While this pattern is familiar, the underlying anatomical reason is frequently misunderstood. The answer lies in the structural constraints of the carpal tunnel and how wrist position changes during sleep increase median nerve compression.
The carpal tunnel is a rigid osteofibrous passage on the palmar wrist. Its floor and walls are formed by the concave arch of the carpal bones, while its roof is the thick, unyielding flexor retinaculum. Inside this confined space lie the median nerve and nine flexor tendons (flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus). Because the boundaries are non-expandable, any increase in volume or reduction in cross-sectional area causes pressure to rise sharply.
During sleep, most individuals naturally allow the wrist to fall into flexion. Even 30–40 degrees of wrist flexion significantly increases intracarpal pressure. This positional change stretches the flexor tendons and their synovial sheaths, causing them to occupy more space within the tunnel. Simultaneously, wrist flexion compresses the median nerve directly against the retinaculum. The result is transient ischemia and conduction block of the nerve’s sensory fibers.
This mechanism explains why symptoms often peak during the night: sustained flexion for several hours compromises median nerve perfusion more than daytime activities, where wrist position changes frequently. Patients with early CTS often have symptoms only at night because daytime movement restores blood flow. As the condition progresses, symptoms may extend into waking hours.
Fluid dynamics also contribute. During sleep, fluid re-distribution from lower limbs increases hydrostatic pressure in the upper extremity. Mild edema in the flexor tendon synovial sheaths can further narrow the tunnel. Pregnancy, hypothyroidism, and rheumatoid arthritis amplify this effect, increasing nocturnal symptoms.
The typical relief achieved by “shaking the hand” has a clear anatomical basis. Brief mechanical movement restores median nerve blood flow, reduces pressure by repositioning the wrist, and temporarily redistributes fluid in the tendon sheaths.
Understanding this anatomy is crucial for management. Neutral-position wrist splinting during sleep remains one of the most effective first-line treatments because it prevents the flexion-induced pressure rise. By maintaining the tunnel’s maximal cross-sectional area, splints significantly reduce nocturnal paresthesia and improve sleep quality.
Carpal tunnel syndrome is ultimately a pressure-dependent neuropathy. Its nocturnal pattern is not random but a predictable outcome of wrist biomechanics, soft-tissue volume changes, and the uncompromising architecture of the carpal tunnel.