The dawn phenomenon is your body's built-in morning alarm going off in your glucose. In the early hours — typically between about 3am and 8am — the body releases a wave of cortisol, growth hormone, and adrenaline to prepare for waking. Those hormones tell the liver to release glucose and make you temporarily more insulin resistant. In people without diabetes, the pancreas quietly covers it; with T1, there is nothing automatic to answer it, so glucose drifts upward before you have eaten anything.
On a CGM it shows up as a slow climb starting in the pre-dawn hours, and it explains two of the most common frustrations in diabetes life: waking up high after going to bed in range, and breakfast spiking harder than any other meal. The same bowl of oatmeal genuinely behaves worse at 8am than at 6pm for many people, because it lands on top of peak insulin resistance.
Worth separating from its look-alike: a rebound after an overnight low (sometimes called the Somogyi effect) can also produce a high morning number. A CGM's overnight trace distinguishes them at a glance — dawn phenomenon is a smooth climb from a flat night, a rebound follows a visible dip.
It varies a lot person to person and week to week — hormones, sleep, and stress all feed it. If your mornings run stubbornly high, the overnight pattern is exactly the kind of thing to review with your care team, since pumps and dosing strategies have well-established ways of addressing it.
