Have problems + Verbo + ING: Como utilizar

Contexto: Scientists found that writing a to-do list helps us sleep faster.

Dúvida: Por favor, porque o verbo fall esta com ing nesta frase?:

"About 40 per cent of adults have problems falling asleep a few times a
Month."

Trecho do a noticia:

"The researchers looked at how students slept. Half of them
Made a to-do list before they slept. Those who wrote a to-do
List fell asleep about 9 minutes faster. Writing detailed lists
Makes you fall asleep faster than writing simple lists. About 40
Per cent of adults have problems falling asleep a few times a
Month."