How to Choose Mattress Firmness UK: Expert Guide 2025

What is Mattress Firmness?

Mattress firmness refers to how soft or hard a mattress feels when you lie on it. It's measured on a scale from 1 to 10, where 1 is very soft and 10 is very firm. Most mattresses fall between 3 (soft) and 8 (firm), with medium firm (around 6 - 7) being the most popular choice for UK sleepers.

Firmness is different from support – while firmness describes the initial feel and comfort of the mattress surface, support refers to how well the mattress keeps your spine properly aligned. A mattress can be soft yet still provide excellent support, or firm but lack proper support. The key is finding the right balance of firmness and support that suits your body weight, sleeping position, and personal preferences.

It's worth noting that mattress firmness is somewhat subjective. What feels medium firm to one person might feel soft to another, especially if there's a significant difference in body weight. This is why understanding your specific needs is crucial when choosing the right firmness level.

Why Does Mattress Firmness Matter?

Choosing the right mattress firmness is crucial for your overall health and wellbeing. The correct firmness level ensures your spine maintains its natural alignment throughout the night, preventing back pain and stiffness when you wake up. When your mattress is too soft, your body sinks excessively, causing your spine to curve unnaturally. Conversely, a mattress that's too firm creates pressure points at your shoulders and hips, leading to discomfort and disrupted sleep.

The right firmness also directly impacts your sleep quality. A properly matched mattress allows you to fall asleep faster and experience fewer sleep interruptions, as your body isn't constantly shifting to find a comfortable position. This means you'll spend more time in deep, restorative sleep phases, waking up feeling refreshed and energized.

Beyond comfort, mattress firmness affects circulation and pressure relief. The ideal firmness distributes your body weight evenly, reducing pressure on sensitive areas and promoting healthy blood flow. This is particularly important for those with joint pain, arthritis, or circulation issues. Getting your firmness right isn't just about comfort – it's an investment in your long-term health and daily performance.

Understanding Spinal Alignment and Mattress Firmness

Your spine's health during sleep is crucial for overall wellbeing. The right mattress firmness ensures your spine maintains its natural alignment throughout the night, preventing back pain and promoting restorative sleep.

The mattress firmness scale is a standardised rating system from 1 to 10 that helps you understand how a mattress will feel. While the scale is industry-wide, it's important to remember that firmness can feel slightly different between brands and mattress types. 

How Different Firmness Levels Affect Your Spine

Understanding how your mattress supports (or fails to support) your spine is the first step in choosing the right firmness level. Let's look at what happens with each type:

Soft Mattress (1 - 3): When a mattress is too soft, your heavier body parts (particularly your hips and pelvis) sink too deeply into the surface. This creates a U-shaped sag in your spine, pulling it out of natural alignment and potentially causing lower back pain. Soft mattresses provide a plush, cloud-like feel with significant sinkage. They're ideal for lightweight side sleepers who need extra cushioning for their shoulders and hips. However, they may lack adequate support for heavier individuals or back and stomach sleepers.

Medium Soft (4 - 5) Medium soft mattresses offer a gentle hug with moderate contouring. They work well for lighter-weight sleepers across all positions and provide good pressure relief whilst maintaining reasonable support. This firmness is popular among side sleepers who want comfort without excessive sinkage.

Medium Firm Mattress (6 - 7): This is the sweet spot for most sleepers. A medium firm mattress provides enough support to keep your spine in natural alignment while offering enough contouring to relieve pressure points. Your spine maintains its healthy S-curve without excessive sagging or arching. Medium firm is the most popular firmness level in the UK, offering an excellent balance of comfort and support. It suits the widest range of sleepers, providing enough cushioning for side sleepers whilst offering adequate support for back and stomach sleepers. Most people with average body weight find this firmness ideal

Firm Mattress (8 - 9): When a mattress is too firm, it doesn't allow your body to sink in at all. This creates gaps between your lower back and the mattress surface, forcing your spine to arch upward unnaturally and creating pressure points at your shoulders and hips. Thus, firm mattresses provide minimal sinkage and strong support. They're best suited for heavier individuals, stomach sleepers, and back sleepers who prefer sleeping "on" rather than "in" their mattress. Firm mattresses excel at maintaining spinal alignment for those who need extra support.

Very Firm (10): Extra firm mattresses offer maximum support with virtually no give. These are relatively rare and typically only recommended for very heavy individuals or those with specific medical requirements. Most sleepers find this level too hard for comfortable sleep.

There are three factors which dictate the firmness of your mattress as below:

Factor 1: Your Body Weight

Your body weight significantly impacts how you interact with your mattress and which firmness level will provide optimal spinal support:

Lighter Build (Under 9 stone OR 59 kg)

• Recommended: Soft to Medium Firm mattresses
• Why: You need enough give to allow the mattress to contour to your body
• Risk: Too firm can create pressure points and prevent proper spinal alignment

Average Build (9 - 16 stone OR 59 - 104 kg)

• Recommended: Medium Firm (ideal for most people)
• Why: Provides the perfect balance of support and comfort
• Benefit: Maintains natural spinal alignment for the majority of sleepers

Heavier Build (Over 16 stone OR 104 kg)

• Recommended: Medium Firm to Firm
• Why: Prevents excessive sinking that can cause spinal misalignment
• Benefit: Ensures proper support throughout the night

Factor 2: Your Sleeping Position

How you sleep has a major impact on which mattress firmness will keep your spine properly aligned:

Side Sleepers

• Need: Soft to Medium Firm
• Why: Your shoulders and hips need cushioning while your waist needs support
• Risk: Too firm creates painful pressure points on shoulders and hips

Back Sleepers ✓

• Need: Medium Firm (optimal choice)
• Why: Supports the natural curve of your spine perfectly
• Risk: Too soft causes lower back to sag; too firm creates a gap under your lower back

Stomach Sleepers

• Need: Medium Firm to Firm
• Why: Prevents hips from sinking and lower back from arching excessively
• Risk: Too soft causes harmful spinal misalignment

Combination Sleepers

• Need: Medium Firm
• Why: Versatile enough to support multiple sleeping positions
• Benefit: You won't need to compromise on support when you change positions

Factor 3: Existing Back Pain or Health Conditions

Lower Back Pain

Medium firm mattresses typically provide the best relief for lower back pain. They support the lumbar region without creating pressure points, allowing your muscles to relax fully during sleep.

Upper Back or Neck Pain

Consider a slightly softer mattress for better shoulder relief, but ensure you pair it with proper pillow support for your neck alignment.

Sciatica

Proper spinal alignment helps reduce pressure on the sciatic nerve. Medium firm support is typically recommended, combined with sleeping positions that minimise nerve compression.

Arthritis or Joint Pain

A medium to medium firm mattress often works best, providing enough cushioning for sensitive joints while maintaining proper support.

No Existing Pain (Prevention)

Focus on prevention with a medium firm mattress that maintains healthy spinal alignment and prevents future issues from developing.

Quick Decision Guide

Use this simple guide to identify your ideal firmness level:

1. Identify your primary sleeping position (side, back, or stomach)
2. Consider your body weight (under 9 stone or 59 kg, 9 - 16 stone or 59 - 104 kg, or over 16 stone or 104 kg)
3. Factor in any existing pain or conditions

Quick Recommendations:

• Side sleeper + lighter build = Soft to Medium Firm
• Back sleeper + average build = Medium Firm ✓
• Stomach sleeper + heavier build = Medium Firm to Firm
• Combination sleeper + any build = Medium Firm

The Sleep Harvester Recommendation

For most people, a Medium Firm mattress provides the optimal balance of support and comfort, maintaining natural spinal alignment regardless of sleeping position. This is why our Medium Firm range is our most popular choice among customers seeking both comfort and spinal health.

Our mattresses are designed with the ergonomics of sleep in mind, providing the support and comfort your body needs for truly restorative rest. Therefore, we have created our luxury mattresses with four comfort options - Soft, Medium Firm, Firm and Very Firm to accommodate every individual's personal needs and preferences. You may like to explore our collection of premium health ergonomic mattresses suitable for all.

Still Unsure? Ask Yourself These Questions:

1. Do you wake up with back pain that improves throughout the day? (You may need firmer support)
2. Do you feel pressure on your shoulders or hips when lying down? (You may need a softer surface)
3. Does your partner have different firmness needs? (Consider adjustable firmness or a medium firm compromise)
4. Do you sleep hot? (Firmer mattresses often sleep cooler with better airflow)
5. How old is your current mattress? (Mattresses lose support over time; if it's over 5 to 7 years old, replacement is likely needed)

Signs Your Current Mattress Isn't Supporting Your Spine

• Waking up with stiffness or pain that wasn't there before bed
• Tossing and turning throughout the night
• Feeling like you need to stretch immediately upon waking
• Sleeping better in hotels or other beds
• Visible sagging or indentations in your mattress
• Your mattress is over 7-10 years old

Take Action for Better Sleep and Spinal Health

Your spine supports you all day long - give it the support it needs at night. Proper spinal alignment during sleep isn't just about comfort; it's about long-term health, pain prevention, and quality of life.

Ready to improve your spinal health and sleep quality? Explore our range of ergonomically designed mattresses at Sleep Harvester, where we combine the science of sleep, sleep research and ergonomic research with superior comfort. Every mattress is crafted with your spinal health in mind, ensuring you wake up refreshed and pain-free.

Sleep better. Live better. With Sleep Harvester - Ergonomics of Sleep, Support & Comfort.

Further Reading

1. Best Mattress Guide UK: Choose Perfect Sleep Solutions 2025 – sleepharvester
2. Cooling Mattresses UK Guide: Best Options for Hot Sleepers – sleepharvester
3. Sleep Optimisation Tips UK: 10 Science-Backed Ways to Sleep – sleepharvester