Your circadian rhythm is an internal clock of roughly twenty-four hours that determines when you are sleepy or alert. It drives your melatonin, cortisol and body temperature, and it is reset every day by light — above all by bright morning light. A stable rhythm is the basis of good sleep; without that rhythm no bed does what it could do.
The circadian rhythm is a biological rhythm of roughly twenty-four hours that drives almost every process in the body: sleeping and waking, hormone release, digestion and core temperature. The central clock sits in the suprachiasmatic nucleus in the hypothalamus and from there drives sub-clocks in every organ.
It is therefore not a habit or a preference, but a physiological mechanism. That also explains why you cannot override it with willpower.
Light-sensitive cells in the retina report to the clock nucleus whether it is day or night. Bright morning light advances the clock and makes you alert; dimmed, warm light in the evening lets melatonin rise. Blue light in the evening does exactly the opposite and postpones your sleep.
In practice that makes morning light the cheapest and most powerful intervention there is: a quarter of an hour outside within an hour of getting up does more for your night than most of the products sold for it.
A bed can only do the work it was made for if the rhythm around it is right. Anyone lying down with a disrupted clock is in effect judging their evening and not their mattress.
That is why a stay at SleepSpa doesn't begin with the bed, but with switching off everything that disturbs the clock: light, screens, alcohol and the hour at which you lie down.
Shifting it, yes; changing it, not really. With consistent wake-up times and bright morning light you can move your rhythm an hour to an hour and a half over a few weeks. Your innate preference — your chronotype — stays underneath it.
Outdoor light, even on an overcast day, is many times stronger than indoor lighting. Ten to twenty minutes outside within an hour of getting up is enough for most people to reset the clock.