That's a lot like our internal psychical experiences. When somber feelings crowd into our psyches, it seems so dark. Maybe your heart rate slows down a bit. Maybe worry and fear dominate. That's when we need the light.
How do you turn the switch to get that light that fills your heart? One way that works: connect with a good friend, explain your mood, ask for advice or consolation.
Another way: get a positive experience through reading. Many people read or recite devotional religious thoughts to let in the light.
I visited a Quaker religious service recently. At one point the leader called for meditative silence. All sounds ceased for several minutes. Then, a few individuals, one after the other, stood to speak about encouraging experiences or moments of divine light. Something like this can happen in any meditative state.
Consider this Hebrew scripture: "The Lord is my light and my salvation." Psalm 27.
Let kids experience darkness and light. Maybe you turn the house lights down low on Good Friday. Then you let them flare brightly on Easter morning, even before people get up from sleep. Invent your own approach together with your children, grandchildren, or, if you're one of the kids, with your parents.
Easter is a great time to let in the light.