Side Sleeping: Benefits, Drawbacks, and How to Do It Right

Evidence-based | Last updated June 2025

Side sleeping is the dominant sleep position worldwide. Surveys consistently find that 54–70% of adults sleep primarily on their side — more than back and stomach sleepers combined. The prevalence isn't accidental: side sleeping naturally keeps the airway open, supports the spine reasonably well, and is comfortable for a wide range of body types. But not all side sleeping is equal. The side you choose, your pillow height, and what you do with your knees all significantly affect how well the position serves your body over the course of a night.

Key takeaway: For most adults, side sleeping — particularly on the left side — is an excellent default position. It keeps the airway open, supports healthy acid reflux management, and is the recommended position during pregnancy. Proper pillow height (4–6 inches for most adults) and a pillow between the knees are the two most impactful adjustments.

Left Side vs. Right Side: Does It Matter?

Both left and right side sleeping keep the airway significantly more open than back sleeping, but there are meaningful physiological differences between the two sides for specific conditions.

Benefits of Left Side Sleeping

Acid reflux and GERD: The stomach sits to the left of midline. Sleeping on the left side positions the gastroesophageal junction above the level of stomach contents, using gravity to keep acid in the stomach. Studies consistently find fewer and shorter acid reflux episodes when sleeping on the left side compared to the right. If GERD is a concern, left side is strongly preferred.

Lymphatic drainage: The thoracic duct — the body's main lymphatic vessel — drains into the left subclavian vein. Some researchers have proposed that left-side sleeping facilitates lymphatic return more efficiently, though direct clinical evidence in sleeping humans is limited. The glymphatic system (brain waste clearance) also appears to function well in lateral positions, though left vs. right differences haven't been firmly established in humans.

Pregnancy: From the second trimester onward, left side sleeping is the recommended position. It reduces compression of the inferior vena cava (the large vein returning blood from the lower body to the heart), maintains uterine blood flow to the placenta, and reduces pressure on the liver. The BUMPS (Born in Bradford) Trial and other obstetric research have reinforced this recommendation. See the position by condition guide for full pregnancy guidance.

Spleen drainage: The spleen, which filters blood and supports immune function, is also positioned on the left side. Left-side sleeping may facilitate splenic blood flow, though this is a secondary consideration for most people.

When Right Side Sleeping May Be Preferred

Heart failure: For people with certain heart conditions — particularly congestive heart failure — right-side sleeping may reduce cardiac workload. The left lateral decubitus position can increase venous return to the heart, which elevates preload. In patients with compromised cardiac function where preload reduction is therapeutically important, right-side sleeping may be more comfortable and less physiologically stressful. This is individual — follow your cardiologist's guidance.

Personal comfort: For people without the above conditions, left vs. right side is largely a matter of comfort. Shoulder pain on one side naturally drives preference to the opposite side. If you sleep with a partner, room layout and habit play large roles.

Acid reflux — nuance: Right-side sleeping is associated with more acid reflux episodes and longer acid clearance times than left-side sleeping. However, it's still significantly better than flat back sleeping for most people with mild GERD. If reflux is a concern, left side is best; right side is a reasonable fallback.

Condition	Left Side	Right Side
Acid reflux / GERD	Best choice	Worse than left, better than back
Pregnancy (after 1st trimester)	Recommended	Acceptable, less optimal
Snoring / sleep apnea	Both sides help equally	Both sides help equally
Congestive heart failure	May increase preload	May reduce preload (often preferred)
Shoulder pain (left shoulder)	Worsens pressure on left shoulder	Better — takes pressure off left
No specific conditions	Slightly more evidence of benefit	Also a good choice

Side Sleeping and Snoring / Sleep Apnea

One of the strongest advantages of side sleeping over back sleeping is its effect on the upper airway. In the supine (back) position, gravity pulls the jaw, tongue, and soft palate posteriorly, narrowing the pharyngeal airway. Side sleeping eliminates this gravitational component — the tongue falls sideways rather than back, maintaining airway patency.

For positional snorers — people who snore primarily on their back and are silent (or near-silent) on their side — switching to sustained side sleeping can be as effective as some snoring devices. Studies suggest 50–60% of people with obstructive sleep apnea have position-dependent OSA, meaning their apnea-hypopnea index (AHI) is at least twice as high in the supine position. For these individuals, consistent side sleeping is a meaningful therapeutic intervention, not just a lifestyle preference.

Position-dependent sleep apnea: If you've been diagnosed with OSA, ask your sleep physician whether your AHI data shows positional dependence. If it does, side sleeping combined with positional training devices may substantially reduce your AHI — potentially reducing or eliminating the need for CPAP in mild to moderate positional cases.

Pillow Selection for Side Sleepers

Pillow height (loft) is the most critical equipment decision for side sleepers. In the lateral position, the pillow must fill the gap between the head and the mattress surface — approximately the width of one shoulder. If the pillow is too thin, the head drops toward the mattress, bending the cervical spine laterally downward. If the pillow is too thick, the head is pushed upward, creating lateral flexion in the opposite direction.

Loft Recommendations

Most adult side sleepers need a pillow with 4–6 inches of loft, but the right number depends on shoulder width and mattress firmness:

Broad shoulders: Wider gap between head and mattress — need higher loft (5–6 inches or more)
Narrow shoulders: Smaller gap — need lower loft (4–5 inches)
Soft mattress: The shoulder sinks in, reducing the gap — lower loft needed
Firm mattress: The shoulder doesn't sink — higher loft needed

A correctly lofted pillow keeps the nose pointing straight forward (not tilted toward the ceiling or toward the mattress) and maintains a neutral cervical spine throughout the night.

Pillow Fill Materials for Side Sleepers

Fill type affects both loft maintenance and adjustability:

Memory foam (solid): Consistent loft, conforms to head shape, retains heat. Good for side sleepers who need consistent height without compression. Look for medium to firm density.
Shredded foam / latex: Adjustable loft (add or remove fill), good pressure relief, less heat retention than solid foam. Excellent for side sleepers who need to dial in the height precisely.
Down / down alternative: Compresses under head weight — tends to go flat for side sleepers unless a high fill-power option is chosen. Often too low for side sleeping without stuffing multiple pillows.
Latex: Supportive and resilient, doesn't flatten, naturally cooling. High-loft latex pillows work well for side sleepers who run hot.
Buckwheat: Adjustable and firm with excellent support, but heavy and not silent. Good for side sleepers who need a consistently firm surface that won't compress.

The Pillow Between the Knees

Placing a pillow between the knees while side sleeping is one of the most consistently recommended ergonomic adjustments for this position. Without it, the top leg drops toward the mattress, pulling the pelvis into rotation and creating lateral flexion stress in the lumbar spine. This is a common source of hip and lower back discomfort attributed to side sleeping that is actually due to poor leg alignment rather than the position itself.

The pillow between the knees keeps the hips stacked — parallel to the mattress — which maintains the lumbar spine in a more neutral position. It also reduces pressure on the hip that's in contact with the mattress by distributing some of the leg's weight.

Thickness: approximately 4–6 inches between the knees is sufficient to keep the hips level. A standard pillow folded in half or a purpose-made contour knee pillow works. Some side sleepers prefer a full-length body pillow that provides both knee separation and something to hug with the upper arms, further stabilizing position throughout the night.

Shoulder Pressure Points

The shoulder is the largest pressure point for side sleepers. The deltoid region — particularly around the greater tubercle and acromion — absorbs the bulk of upper body weight in the lateral position. Over a 7–8 hour night, this sustained pressure can cause:

Shoulder pain or soreness on waking (usually resolves within an hour)
Numbness or tingling in the arm (compression of brachial plexus or axillary blood flow)
Aggravation of existing rotator cuff pathology or shoulder bursitis

Key factors that reduce shoulder pressure:

Mattress pressure relief: A mattress that allows the shoulder to sink in appropriately (rather than pushing back against it) reduces peak pressure. Softer comfort layers (2–3 inches of foam or latex over a firmer core) are well-suited to side sleepers.
Arm position: Keeping the bottom arm roughly in front of the body rather than tucked under the torso reduces direct compression on the shoulder joint. Reaching the arm straight up or forward (yearner style) can reduce acromion pressure by shifting the contact point.
Alternating sides: Switching which shoulder takes the weight regularly distributes wear over time.
Body pillow: Hugging a body pillow with the upper arm keeps the top arm forward without pressure, reducing tendency to roll onto the bottom shoulder.

If shoulder pain persists after optimizing these factors, see a physician. Persistent rotator cuff or bursa pathology can be worsened by sustained lateral compression and may require targeted treatment independent of position optimization.

Side Sleeping, Skin Aging, and Sleep Lines

Side sleeping creates sustained mechanical compression of facial skin against the pillow. Over years and decades, this contributes to:

Sleep lines: Temporary creases in the skin from pillow compression, visible in the morning. In younger skin these resolve within minutes; in older skin with reduced collagen density, they may persist for hours or become partially permanent.
Asymmetric facial aging: Studies examining facial photographs of long-term side sleepers find measurable asymmetries in skin laxity, wrinkling depth, and cheek volume on the preferred sleep side versus the non-sleep side. A 2012 study in Aesthetic Surgery Journal by Guyuron et al. on identical twins found consistent differences in cheek and periocular wrinkling corresponding to habitual sleep side.
Décolletage creasing: Similar compression patterns affect chest skin in women.

Reducing Skin Impact

Complete prevention requires avoiding side sleeping (not practical for most people). Mitigation strategies:

Silk or satin pillowcases: Lower friction coefficient reduces mechanical tugging on skin compared to cotton. The skin slides rather than catching and bunching, which reduces crease depth. Silk also retains less moisture, which may benefit some skin types.
Copper-infused pillowcases: Marketed for skin benefits; some small studies suggest copper oxide fabric may support collagen synthesis. Evidence is limited but these are low-risk if you're interested.
Back sleeping: Eliminates facial compression entirely — the main reason back sleeping is often called the "beauty sleep" position by dermatologists.
Alternating sides: Distributes the compression bilaterally, reducing asymmetric aging rather than eliminating total impact.
Nighttime skincare: Adequate moisturizer supports skin barrier function but doesn't prevent mechanical compression effects.

Realistic perspective: Skin aging from sleep position is a cumulative, decades-long process. It's one factor among many (sun exposure, genetics, smoking, diet). For most people, the health benefits of side sleeping far outweigh cosmetic concerns. If skin aging from sleep position is a priority, silk pillowcases and alternating sides are practical first steps before considering position changes.

Common Side Sleeping Problems and Fixes

Problem	Likely Cause	Fix
Neck pain on waking	Pillow too low or too high	Adjust pillow loft to match shoulder width; try shredded fill for adjustability
Lower back pain	No pillow between knees; pelvis rotating	Add knee pillow; try body pillow to stabilize position
Shoulder soreness	Insufficient mattress pressure relief; arm under torso	Add soft topper; adjust arm position forward; alternate sides
Arm numbness/tingling	Brachial plexus compression; arm tucked under body	Reposition arm forward; use lighter, properly lofted pillow
Hip pain	Mattress too firm; no knee pillow	Add knee pillow; consider mattress topper for pressure relief
Rolling to back during night	No positional anchor	Body pillow behind back; positional alarm device
Sleep lines / skin creases	Cotton pillow friction	Switch to silk or satin pillowcase; alternate sides

Who Benefits Most from Side Sleeping

Side sleeping is particularly well-suited for:

Snorers and sleep apnea patients (especially positional apnea)
People with GERD or acid reflux (left side strongly preferred)
Pregnant women from the second trimester onward (left side)
People with lower back pain who find back sleeping uncomfortable — with knee pillow
Most adults without specific conditions as a reliable, well-tolerated default

Frequently Asked Questions

Is sleeping on your left side better than your right?

For acid reflux, GERD, and pregnancy, left side sleeping is clearly better supported by evidence. For snoring and sleep apnea, both sides are roughly equivalent — what matters is being off your back. For people with heart failure, right-side sleeping may be preferable to reduce cardiac preload. For healthy adults without specific conditions, the difference is small and comfort should guide you. Alternating sides distributes pressure and is a reasonable habit regardless of which side you prefer.

What height pillow do I need for side sleeping?

Most adult side sleepers need a pillow with 4–6 inches of loft. The exact height depends on shoulder width (broader shoulders need more height) and mattress firmness (softer mattresses allow the shoulder to sink, requiring less pillow height). The goal is keeping the head in line with the spine — nose pointing straight forward, neck neither bent up nor down. If you wake with neck stiffness, adjusting pillow height is the first thing to try.

Why do I wake up with shoulder pain from side sleeping?

Shoulder pain from side sleeping is usually caused by either: sustained pressure on the deltoid and shoulder joint from a mattress that doesn't provide adequate pressure relief, a pillow that's too high (pushing the shoulder into an elevated position), or having the arm tucked under the body overnight. Try adjusting your arm position forward (extend it out from the shoulder rather than under your torso), switch to a mattress with softer comfort layers, and make sure your pillow height is appropriate. If pain is severe, persistent, or involves significant numbness, see a physician to rule out rotator cuff injury or shoulder bursitis.

Does side sleeping cause wrinkles?

Side sleeping can contribute to facial wrinkles and asymmetric skin aging over time, due to sustained mechanical compression of the face against the pillow. Research on identical twins confirms measurable sleep-side facial asymmetries in long-term side sleepers. That said, this is a slow cumulative process — decades of one-sided sleeping. Practical mitigation includes silk or satin pillowcases (reduce friction), alternating sleep sides, and considering back sleeping if facial aging is a high priority. The health benefits of side sleeping for most people outweigh cosmetic concerns.

Should I use a body pillow for side sleeping?

A body pillow is particularly useful for side sleepers who: roll onto their back during the night (the pillow placed behind acts as a physical barrier), have lower back or hip discomfort (the pillow supports both knees and provides something for the upper arm to rest against), are pregnant (maintaining left-side position), or have shoulder issues (the top arm rests forward on the pillow rather than falling inward). They're not necessary for everyone, but are a low-cost, effective solution for the above scenarios.

This content is for educational purposes only and does not constitute medical advice. If you have shoulder pain, sleep apnea, GERD, cardiovascular conditions, or are pregnant, discuss sleep position with your healthcare provider.