Sleep is a heat-transfer problem before it is a discipline problem.
Your body cannot enter NREM sleep unless core temperature is actively falling. LongevityStack treats that as an engineering problem: the mechanism first, then the evidence from primary sources.
Four places the heat has to pass through before you fall asleep.
Every page on this site works on one of these stages. Each links to the section of the mechanism that sources it.
Core temperature has to be falling. Sleep onset is most likely when that decline is fastest, not when it bottoms out.
Hands and feet dump the heat. In one controlled study, the distal–proximal skin gradient outpredicted core temperature, heart rate and melatonin timing for how fast people fell asleep.
Bedding is the first environment the skin sheds heat into. Same rule as the room: heat only leaves if there is a gradient to leave into.
The room is the heat sink. The often-quoted 18–19 °C comes from studies of young adults; the parameter that matters is the gradient, not one number.
Peer-reviewed studies linked by DOI. Evidence blocks state study type, sample size, population and funding.
Every mechanism page has a section on the open questions and the limits of the evidence.
Every mechanism page shows when its sources were last checked. How evidence is graded.