Why the brain's clock, not the melatonin source, is the target
The suprachiasmatic nucleus is a small cluster of neurons in the hypothalamus that acts as the body's master circadian clock. Melatonin is a hormone released by the pineal gland, but the receptors that respond to it are found in the SCN [1]. When a melatonin agonist binds those receptors, it can nudge the timing of the clock itself, which is why the effect is described as receptor-mediated phase shifting within the SCN [1].
This distinction matters for the question. The pineal gland produces melatonin, but the evidence does not show that the pineal is required for a melatonin agonist to speed re-entrainment. Instead, the SCN is where the relevant receptors sit and where the phase-shifting signal is thought to act [1]. One review notes that the SCN controls rhythmic melatonin release from the pineal gland, which is a separate relationship from the agonist's direct effect on the clock [2].
What speeding up re-entrainment actually looks like
Re-entrainment is the process of a biological rhythm catching up to a new light-dark schedule after a phase shift, such as after traveling across time zones or switching to night shifts. In mouse models, researchers measure how quickly rhythmic behaviors such as wheel-running adjust to the new schedule, and they use these models to test how effectively melatonin receptor ligands shift rhythms and speed re-entrainment [1].
A melatonin agonist has been reported to produce rapid re-entrainment following a phase shift in a clinical context [4]. That observation is consistent with the idea that activating melatonin receptors can accelerate the clock's adjustment, though the report does not specify whether the SCN or pineal gland was involved [4]. The mouse paradigms described for assessing melatonin receptor-mediated phase shifts and re-entrainment provide the controlled setting in which this kind of acceleration can be quantified [1].
Does the pineal gland need to be involved?
The evidence does not support a requirement for the pineal gland in the re-entrainment effect. The SCN is identified as the site of MT1 and MT2 receptor expression through which melatonin and related agonists phase-shift circadian rhythms [1]. The pineal gland is discussed as a source of melatonin under SCN control, not as a necessary intermediary for the agonist's effect on re-entrainment [2].
Some studies of melatonin agonists and phase shifting have been done in birds with intact pineal glands, and those studies report phase-shift responses to a melatonin agonist [3]. That work shows the agonist can elicit a phase shift in a pineal-intact system, but it does not establish that the pineal gland is required, and it does not address the SCN directly [3]. The more direct evidence for the SCN as the relevant site comes from the receptor localization and mouse paradigms [1].
What the evidence leaves open
The picture is built from methods papers, reviews, and abstracts rather than a single definitive experiment that removes the pineal gland and tests re-entrainment. The mouse models are described as predictive measures of therapeutic efficacy for circadian phase disorders, which means they are designed to test whether a ligand can shift rhythms and speed re-entrainment, not to settle the anatomical question on their own [1].
The clinical report of rapid re-entrainment with a melatonin agonist does not specify the involvement of the SCN or pineal gland, so it supports the behavioral effect without resolving the mechanism [4]. Taken together, the evidence is consistent with an SCN-mediated effect that does not require the pineal gland, but the strongest support for that conclusion is the receptor localization in the SCN rather than a direct pineal-removal experiment [1][2].
Sources used in this answer
Methods to Assess Melatonin Receptor-Mediated Phase-Shift and Re-entrainment of Rhythmic Behaviors in Mouse Models
Melatonin and related agonists phase-shift circadian rhythms and promote re-entrainment in mouse models via MT1 and/or MT2 receptors expressed in the suprachiasmatic nucleus.
Essay: Rhythmic melatonin release controlled by the SCN and its consequences for glucose homeostasis
The suprachiasmatic nucleus controls rhythmic melatonin release from the pineal gland, and phase shifting and re-entrainment are discussed in that context.
Regulation of sleep and circadian rhythms
A single injection of the melatonin agonist 6-chloromelatonin elicited a phase shift in pineal-intact birds.
11. Range of Chronotherapeutic Indications
A melatonin agonist was associated with rapid re-entrainment following a phase shift, without explicit detail on the suprachiasmatic nucleus or pineal gland.