your betta is losing color. OK, here is a new concept for a few of you. Betta colors are organized in layers, and some layers are always at the top. For example, black covers red. So a betta might be red, with a black layer on top. Sorta think of it as a wall that has been painted several times. One coat or red paint, then a coat of black paint. Now, if you do not know there is a red coat underneath the black, you would think it is just a black wall. But peel off the black layer of paint and you will see red underneath. Likewise if your betta has a red layer under his black layer, and if, when he sleeps, his black pigment becomes less intense (a normal betta thing), you would then be able to see the red layer peaking 'below'. Hence making his head look now 'red'. Could be one explanation.

Other option: you are looking at your betta with artificial light (flash light, table top light, any light source) at night. The light may refract and show a different color. For example, and as an interesting experiment, I want all of you tonight to go get a flashlight (not TOO powerful please, we don't want to make the bettas blind now) and to gently shine it on your bettas. You will be amazed how different some of them will look under a flashlight! Primarily a lot more blue or green iridescence will appear, some you never even know was there!