I have spoken to many Buddhists who say that when they pray at night they do it to "sing sud sid".

Some others have told me that this is a Hindu higher power.

I am confused. Do Thai Buddhists pray to a Hindu God?

if I read your phonetics correctly it means

sing = thing

sak sit = holy

i.e. a holy thing to do

I have spoken to many Buddhists who say that when they pray at night they do it to "sing sud sid".

Some others have told me that this is a Hindu higher power.

I am confused. Do Thai Buddhists pray to a Hindu God?

That's สิ่งศักดิ์สิทธิ์ (sing[L] sak[L]sit[L]) or 'holy things'. It can refer to anything from Buddhas and Buddha amulets to ceremonial water blessed by monks to animist charms.

Many Thais simultaneously recognise Buddhist deities (the various Buddhas, plus Bodhisattvas like Phra Upakhut/Sri Upagupta), Brahmanist deities (chiefly Brahma, Indra and Shiva) and animistic spirit entities (land, water and plant spirits, not to mention a host of humanlike ghosts).

I've heard people say they attend to things Buddhist to take care of their future lives, things animist and Brahmanist for their present lives. It doesn't exactly go along with orthodox Buddhism but that's the way it is for most Thai Buddhists.