All electric light fluctuates. The question is by how much and how fast. Flicker is measured as a percentage, and cheap LED drivers often produce deep flicker at frequencies the conscious mind does not register but the visual system still processes. Many people tell us they notice a difference in how a room feels once flicker is reduced, particularly in spaces where they read or work. Look for bulbs that publish a flicker figure - most do not, which tells you something.

Colour temperature, measured in kelvin, is a rough proxy for how much blue is in the mix. Below 2700K is warm and evening-appropriate. Around 5000K is closer to midday daylight. A bulb that stays at 5000K after sunset is giving your body a signal that contradicts the time on the clock.

The colour rendering index, or CRI, describes how faithfully a light source shows colour compared with a reference source, on a scale up to 100. Standard LEDs frequently sit in the 70s or low 80s. Skin, food and natural materials all look flatter under low CRI light. Above 95 is where things start to look the way they do outdoors.

This one is rarely discussed. Many LED bulbs contain switching drivers that put high frequency voltage transients back onto the household wiring. Magda Havas at Trent University in Canada has researched this phenomenon for years. It means a lighting decision is also an electrical environment decision, which is why we measure both together.

Bright and full spectrum

Get outside if you can, even for a few minutes, because outdoor light at dawn is an order of magnitude stronger than anything indoors. Where that is not possible, full spectrum bulbs in the rooms you use first give you the closest indoor equivalent.

Neutral and generous

This is when brightness genuinely helps. Working under dim light in the middle of the day asks your visual system to do more work than it needs to.

Begin warming

As the sun drops outside, let the light indoors follow it. Lamps rather than overheads. Lower colour temperatures.

Dim and red-shifted

Table lamps, candles, salt lamps, red bulbs. Low and warm rather than overhead and bright. This is the window most people find makes the biggest difference to how they feel by bedtime.

Dark. Genuinely dark.

Standby lights, street lighting through curtains and phone screens all count.

The most important room to get right. Warm, dim, low-level light in the evening. Blackout at night. Avoid anything blue-rich after sunset. Red bulbs in bedside lamps are an easy change with a noticeable effect.

Full spectrum and high CRI during the day, because you are preparing food and want to see it properly. A separate warm circuit or a lamp for the evening - using BioLights provides both options as needed.

Layered rather than uniform. Several warm sources at different heights beat one bright overhead. Avoid dimmer switches (dirty electricity) and use BioLights to provide a full range of light options .

High CRI, flicker-free, positioned to avoid glare on screens. This is the room where flicker and poor colour rendering are felt most, because you are under the light for hours.

Bright enough for the morning, with a low warm option for the evening. Bathrooms are often the last bright room before bed, which quietly undoes an evening of careful lighting elsewhere.

The same principles, applied a little more firmly. Warm and dim in the evening, dark at night, and a red night light rather than a white one where one is needed.