JavaScript prototype access variable in function subfunction

I am trying to access variables from the constructor inside a sub-function of a prototype defined function. I understand that this.variable can access constructor variables in prototype functions but it cannot be accessed from another function inside.

function MyObject() {
    this.message = "Hello world!";
}
MyObject.prototype = {
    sayHello: function() {
        console.log(this.message);
    },
    repeatHello: function() {
        setInterval(function() {
            this.sayHello();
        }, 1000);
    }
}
let test = new MyObject();
// test.sayHello();
test.repeatHello();

With the example above, since I am calling this in the new function it makes sense it can't read the functions and variables outside of it since they are not defined to that function. How do I get access to sayHello() or this.message from the sub-function?