Why the Same Meal Gives Different Readings

You eat the same breakfast you always eat, and the reading is forty points higher than yesterday. Nothing has gone wrong — several things other than food decide that number.

What changed, even though the meal did not

• Last night's sleep. Even one night of partial sleep deprivation measurably increases insulin resistance. Sleep restricted to 6.5 hours raised post-meal glucose by around 14% in controlled work.
• The time of day. The body handles the same food differently morning versus evening — in circadian experiments, deliberately shifting timing raised post-meal glucose by about 6% with identical meals.
• Stress. Stress hormones raise glucose directly.
• Yesterday's activity. Exercise improves insulin sensitivity for many hours afterwards, so a walk yesterday can lower today's reading.
• Illness, even a cold you have not noticed yet.
• How fast you ate it, and what you were doing afterwards.

This is a different question from why *different* meals spike differently — that one is here.