Sleep is usually poor before surgery, worse for a few weeks after, and better by six months.

Sleep is poor before surgery and improves later, not at once

In a prospective study of 780 hip and knee replacements, 35 percent of patients reported poor-quality sleep before surgery. It was more common before hip replacement, at 39 percent, than before knee replacement, at 30 percent. Of those who slept badly beforehand, 74 percent reported better sleep a year after surgery ¹.

The improvement is not immediate. A meta-analysis of sleep-quality scores after knee replacement found little change from baseline at 4 to 6 weeks. It found a significant improvement at 3 to 6 months. The included studies reported disrupted sleep close to surgery, especially the first days, and better sleep at every later interval ².

So the pattern most patients describe, a few weeks of worse nights before things turn, is the one the studies measured. Sleep still poor months later is a different matter and worth raising, since the trials expected improvement by then ².

After hip replacement, the position depends on the approach and the surgeon

The AAOS patient guide says that, depending on the surgical approach, a surgeon may ask a patient to avoid certain sleeping positions. Some are asked to sleep with a pillow between the legs for a period. The guide tells patients to ask their own surgeon which positions are safest ³.

Behind this is the traditional set of hip precautions after a posterior approach. They are no crossing of the legs, no bending the hip past 90 degrees, and no turning the leg inward. They usually apply for the first 6 to 8 weeks ³. Anterior-approach protocols often carry fewer restrictions, so two patients with the same operation can be given different sleeping advice.

The evidence behind precautions has shifted. A 2023 meta-analysis pooled 4 randomised trials and 5 non-randomised studies with 8,835 patients. It found no or negligible difference in early dislocation with or without precautions after the posterior approach. In the trials, 1.2 percent with precautions dislocated within 3 months against 0.6 percent without. The certainty of the evidence was rated low, and the authors concluded it does not support routinely prescribing precautions ⁴.

Some surgeons have dropped precautions on that basis; others keep them, because the trials are few and a dislocation is serious. Which set applies to you is your surgeon's call from how the hip was reconstructed and how stable it was in surgery. Follow the version you were given, not the one a friend was given.

After knee replacement, any position is fine, but not with the knee propped bent

The AAOS guide states that after knee replacement it is safe to sleep on the back, on either side, or on the stomach ⁵.

The pillow is a different question. The goal in the early weeks is a knee that straightens fully. The AAOS exercise guide works toward it with a small rolled towel above the heel. The heel is off the bed and the knee hangs straight ⁶. A pillow under the knee holds it bent for hours, the opposite of that exercise. A knee that will not fully straighten, called a flexion contracture, is a recognised complication after knee replacement. Maintaining full extension during rehabilitation is a stated priority in the surgical literature ⁷.

Sleeping position itself may matter. In a series of 150 knee replacements, back sleepers lacked a mean of 2.98 degrees of full extension at one month. Side sleepers lacked 6.03 degrees. The authors note that 5 degrees is the threshold usually cited for normal walking mechanics, and that patient-reported outcomes did not differ ⁸. It is one study from one surgeon; it argues for attention to a straight knee, not against side sleeping.

Small things that help the nights

• After knee replacement, keep the knee straight in bed, with any support under the ankle rather than the knee ⁶.
• After hip replacement, keep whichever precautions you were given, including any pillow between the legs, for the period your surgeon set ³.
• Expect the first weeks to be uneven, and judge progress over weeks rather than nights ².
• Take prescribed pain relief and do the day's exercises exactly as the surgical team directed. Changing that plan is their decision, not a bedtime experiment.

When poor sleep should prompt a call

Most bad nights after joint replacement are pain and position, and they ease. A few patterns are not sleep problems but something else showing up at night. The AAOS guides list the ones to report ³⁵:

• Calf or leg pain unrelated to the incision, tenderness or redness of the calf, or new swelling not relieved by elevation ³⁵.
• Sudden shortness of breath or chest pain, which needs immediate medical attention ⁵.
• Fever above 100 degrees, chills, or increasing redness, drainage or pain at the wound ³.
• After knee replacement, a knee that is losing straightening rather than gaining it, since extension is easier to keep than to recover ⁷⁸.
• Sleep that is no better, or worse, months after surgery, when the studies expected improvement ².