The gunas do not take fixed seats. They are constantly jostling for dominance. Sometimes sattva overpowers rajas and tamas. Sometimes rajas pushes the other two aside. And sometimes tamas swallows up both sattva and rajas.
Watch clouds moving across the sky on a windy day. One moment the sun breaks through, clear and warm. A minute later, clouds roll in and everything turns grey. Then the wind shifts again and light returns. The mind works exactly like this. All three gunas are always present, but only one holds the stage at any given moment.
This insight is freeing. When you notice tamas creeping in — the heaviness, the fogginess — you can understand it as a temporary shift, not a permanent condition. The gunas are always in motion. What dominates now will not dominate forever.