Why You Shouldn’t Use Your Smartphone in Dim Lighting: Understanding Screen Brightness and PWM Dimming

We’ve all done it—scrolling through social media in bed with the lights off, checking emails late at night, or watching videos in a dark room. While these habits might seem harmless, using your smartphone in dim light is more problematic than you might think. The combination of low screen brightness and a display technology called PWM dimming is particularly straining on the eyes in low-light environments.

Understanding PWM Dimming: The Hidden Flicker

Pulse Width Modulation (PWM dimming) is a technique most OLED smartphone manufacturers use to control screen brightness. Instead of continuously lowering the actual light output, it leverages the persistence of vision in the human eye by rapidly switching the screen pixels between “on” and “off” states—sometimes hundreds or even thousands of times per second—to achieve the desired effect. Your eyes perceive this rapid flickering as dimmer light, similar to how a strobe light appears less bright at lower flicker frequencies.

This method offers several advantages for screens:

• Power Saving: Compared to analog dimming, which requires continuous power to maintain different brightness levels, PWM can potentially utilize energy more efficiently by precisely controlling the “on” time percentage to achieve the same perceived brightness.
• Precise Control: PWM signal generation and control are relatively easy to implement, allowing for precise brightness adjustments across a wide range of luminance levels.
• Color Accuracy: Especially at low brightness, PWM dimming can better maintain the light emission color of OLED pixels compared to DC dimming (which directly reduces the driving current), avoiding color shifts. This is because pixels are either fully on or off, and their color when lit is fixed.
• Wider Dimming Range: PWM dimming can theoretically achieve very low brightness levels, approaching a completely off state.

However, manufacturers don’t always emphasize that this flickering, while often not consciously perceived, can have real physiological effects on users. The on-off cycle frequency varies by device and manufacturer, with some devices operating at frequencies as low as 200-300 Hz.

Why Dim Light Makes Everything Worse

Dim ambient light combined with PWM dimming creates a problematic combination for eye health. When using your smartphone in such an environment, your pupils naturally dilate to allow more light in. This biological response makes your eyes even more sensitive to the rapid brightness changes caused by PWM dimming.

Furthermore, in low-light conditions, you are more likely to set your screen to a lower brightness level to avoid discomfort from an overly bright display. However, many smartphones implement more aggressive PWM dimming at lower brightness settings. This means that precisely when your eyes are most sensitive to flickering, the flickering becomes more pronounced.

The contrast between the dim environment and the flickering screen also forces your visual system to work harder. Your eyes (including your pupils, retina, and visual cortex in the brain) are constantly adjusting between the surrounding darkness and the pulsating light emitted by the device, perpetually trying to adapt to these light changes, leading to various symptoms.

It’s prudent to use your smartphone or any screen-based device in well-lit environments. Adequate lighting reduces the need for lower screen brightness, consequently reducing the risk of eye strain.

How Human Eyes Respond to Flickering

While most people cannot consciously notice PWM flicker, our visual system still registers these rapid changes in light intensity. Studies show that even flicker frequencies far above our conscious perception can trigger various physiological responses. Some individuals are particularly sensitive to this phenomenon, experiencing symptoms ranging from mild discomfort to more severe issues.

The effects of PWM flicker can manifest as:

• Eye Strain and Fatigue: This is one of the most common symptoms. The eyes constantly adapt to changes in brightness, and even if these changes are too fast for conscious detection, eye muscles like the ciliary muscle can overwork, leading to a feeling of fatigue.
• Dizziness or Nausea, even Headaches: In more sensitive individuals or cases with severe flicker, some may experience dizziness, or even mild nausea. PWM flicker can also induce or worsen headaches, sometimes even migraines.
• Dry Eyes: When staring at a screen for a prolonged period, the blinking rate may decrease. PWM flicker can exacerbate this condition, leading to dry eyes, a burning sensation, or a gritty feeling.
• Anxiety: Persistent visual discomfort and the resulting physical symptoms can indirectly lead to emotional anxiety.
• Photophobia/Light Sensitivity: Reduced tolerance to light, where screen light or ambient light feels harsh.

Alarmingly, many people don’t associate these discomforts with PWM dimming. They might attribute the symptoms to general “screen time” or believe that feeling tired after using their phone is normal, without realizing that the flickering itself could be the primary cause.

Sleep Quality and Circadian Rhythms

Beyond immediate discomfort, there are concerns about the long-term effects of regular exposure to PWM flicker, especially under suboptimal lighting conditions. While research is still evolving, some studies suggest that prolonged exposure to certain types of artificial light flicker can lead to visual fatigue and potentially affect sleep patterns.

The blue light emitted by smartphone screens has already been linked to circadian rhythm disruption, and the combination of PWM flicker with inappropriate lighting conditions may exacerbate these effects. Using your phone in dim light often means you’re doing so at night, which is precisely when your body should be preparing for sleep.

Practical Solutions for Better Screen Habits

The following tips can help you effectively reduce the negative impacts of PWM dimming and improve your overall screen usage experience:

1. Ensure Ample Ambient Lighting Avoid completely dark environments when using your smartphone. You don’t need harsh, bright lights, but rather soft, consistent illumination to reduce the stark contrast between your screen and your surroundings.
2. Invest in Adjustable Lighting Consider equipping areas where you frequently use your phone with dimmable light fixtures. For example, a dimmable bedside lamp can provide enough ambient light at night to make phone use less straining without disrupting your sleep preparation.
3. Enable DC Dimming (If Available) Some smartphone manufacturers offer a DC dimming option in their display settings. This method reduces brightness by actually lowering the backlight intensity rather than using PWM. If your device supports this feature, enabling it can significantly reduce flicker-related eye strain.

These simple adjustments can help protect your eye health and enhance overall comfort when using your screen.

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By understanding the mechanism of PWM dimming and its effects in dim light, you can take proactive steps to reduce discomfort and protect your well-being. The next time you’re scrolling on your phone late at night, consider turning on a small light—your eyes will thank you!