Keep in mind that cortisol is a major player in metabolism and it cycles throughout the day, I believe they call it diurnal meaning levels fluctuate throughout the day/night. This explains the late afternoon crash of 'normal' people, I quoted the word
normal since many ME people have disturbed circadian rhythms so their sleep/wake cycles are disturbed meaning that crashes come in different times for ME. Much of this comes from the dysregulated HPA axis. Adrenals play the role of secreting cortisol and other hormones.
B12 can act as a simulator of metabolism and I have heard some people crashing on it. B vitamins are a big happy family and when one gets too strong, the others can become depressed. All B vitamins participate in different layers of metabolism. Vitamins B2 and B6 are particularly helpful for me, they can support adrenals as well as immunity. But that case is for me, I happen to take a balanced b complex and will supplement from time to time with B2 and B6.