Why Does Shoulder Pain Get Worse at Night?

Understanding the Circadian Biology of Frozen Shoulder and Rotator Cuff Pain

Ask anyone with a frozen shoulder or rotator cuff tear, and you’ll likely hear the same thing:
“The pain is worse at night.” “I would do anything for a good night’s sleep”.

Why is this the case? There are a several hypotheses.

Let’s start with the most intuitive. At night, there are fewer distractions. The absence of background noise, screen time, and daily busyness can sharpen awareness of bodily discomfort. Add to that the simple biomechanical reality: lying down places pressure on the shoulder and alters joint orientation, potentially irritating already sensitised structures like the joint capsule or subacromial bursa.

Another likely factor is inflammation. Many shoulder conditions involve low-grade inflammatory processes that ebb and flow. Since inflammation is modulated by circadian rhythms, it may peak at night, contributing to a baseline increase in sensitivity.

But beyond these familiar ideas, recent research has introduced a new player into the discussion: melatonin.

Melatonin is the hormone that regulates sleep-wake cycles. It rises in the evening and stays elevated overnight, plummeting in the morning to signal wake time (yay). Melatonin also has functions outside of sleep-wake cycles, it also influences inflammatory and nociceptive pathways. In people with frozen shoulder and rotator cuff tears, studies have shown increased melatonin receptor expression in the shoulder capsule and bursa. Why? We’re not sure.

These receptors stimulate ASIC3 (Acid-Sensing Ion Channel 3), which sensitises nociceptors to chemical changes in inflamed tissue. Melatonin also increases IL-6, a pro-inflammatory cytokine linked to pain. In short: more melatonin, more noxious stimuli, often more pain.

This may partially explain why shoulder pain seems to spike during the night: the body is biologically primed for it.

So, is night pain mechanical, inflammatory, or hormonal?

Probably all three.

Understanding the multiple contributors, from sleep posture to circadian biology, allows clinicians to take a more nuanced approach to educating about night pain. This might involve discussing sleep hygiene, modifying sleeping positions, timing treatments strategically (chronotherapy), or simply acknowledging to patients that their pain has a rhythm.

Reference

1. Ha, E., et al., Melatonin Plays a Role as a Mediator of Nocturnal Pain in Patients with Shoulder Disorders.J Bone Joint Surg Am, 2014. 96(13): p. e108.