The psychological /mental symptoms of PMS and PMDD are thought to be primarily due to changes these hormones induce in three main brain transmitter systems: these brain chemicals include opioid, GABA (gama-aminobutyric acid), and the serotonin systems. Think of transmitters as keys that when inserted into their proper locks will turn on certain brain functions. If there are not enough "keys" these functions cannot be turned on very promptly, if there are jammed locks, then the function won't ever occur. It's very complex as sometimes it's not the hormone that triggers the action, but one of its metabolites (as they get metabolized they get into other hormones). So the actual outcome of what the brain effect has to do with your hormone level, how much it's been metabolized, and your natural levels of the brain transmitters.
Fatigue is often an important component of the PMS and PMDD complex and Some menstrual fatigue is due to a mild degree of anemia or deficiency of other vitamins do to the blood loss of menstruation. Some menstrual fatigue and the disordered thinking women get with their cycles (and fog) is due to the disruption that is produced by headaches and poor sleep.In addition some of these side effects can be worsened by those treatments that make other aspects of PMS better. Often gyno docs will treat PMS with progesterone. Progesterone in high doses makes one sleepy, so this might contribute in the second half of the cycle.
Sugar metabolism disorder can also make one think and function poorly. The hormones of the menstrual cycle affect the effectiveness of insulin's ability to direct the processing of sugar, so the highs are higher and lows are lower, we think this may also be related to cortisol, but cortisol has more of a daily variation rather than closely linked with our cycles. This is the main reason to avoid sugar. But too much sugar intake can cause gas, diarrhea, and water retention so to a large extent the effects of sugar on our mental functions is not severe, but the effects of sugar on the physical part of PMS can be important. Gynos think that the mental symptoms that are mood or anxiety related may be related to changes in serotonin, and the thinking may be more related to the GABA system or the opioid system.
Simple therapies for fatigue and foggy thinking are useful no matter what the cause: better rest, iron, good nutrition, exercise. Ultimately changing the neurotransmitters may be the best way to fight brain fog. You can preload opiods earlier in the cycle by being a high exerciser in that 2nd and third week before PMS/PMDD hits in the third week.. Long term and the more fit exercisers also have steadier and overall lower estrogen levels so they won't quite have the peaks and valleys of hormones, so that may be why this works. Cardio and strength building both help release endorphins as does yoga. The breathing exercises of yoga also help pace our physiologic rhythms and improve thought processing.