Twenty to thirty minutes of natural light every morning. That is what biologist Maria Jose Martinez, chief executive of Chronohealth, asks for to get working the system that regulates your sleep, hormones, metabolism, immunity and concentration. Not a supplement, not an app, not a device. Going outside.

Her comparison is blunt: "Natural light is to our biological clock what food is to metabolism - a basic need." Light entering the eye activates a biological system that synchronises the circadian rhythms, those twenty-four-hour cycles by which the body knows when to be awake and when to shut down.

The morning is not chosen at random. Martinez calls it the body's "reset button" - the moment the internal clock aligns with the actual day. Miss it, and the body spends the whole day running on a clock showing the wrong time, and the consequences are not just feeling sleepy at night.

The key is in the wavelength. Blue light, most abundant in the morning, switches on alertness. Warm yellow and red tones in the evening let the body produce melatonin and get ready for sleep. The problem arises when the order gets flipped - which is what we do every evening.

Bright screens and intense LED lighting before bed send the brain one single message: it is still daytime. The brain accepts it, because it has no reason not to - for millions of years light like that meant the sun was up. Martinez is clear: avoid bright screens and long light exposure before going to bed.

Autumn and winter demand a little more time outside, because the light is weaker. Here that means from October onwards the same effect requires a longer stay outdoors - precisely in the period when we least feel like going out.

What makes this recommendation unusual is that nothing is being sold. No capsules, no subscription, no device to measure it all. Maybe that is exactly why it gets far less airtime than the latest sleep supplement.