Serenity Strull/ BBC/ Getty Images In ancient times, parents probably didn't suffer as much from feeling sleep deprived, leading some scientists to reconsider guidelines for today's new parents to get some shut-eye. How exhausting is it to be a parent? When one brave soul posed this to a Reddit parenting forum, more than 400 answers flooded in . "Extremely. It's extremely exhausting and it is literally CONSTANT," wrote one respondent, the sentiment echoed by hundreds of others. There are many reasons why parents feel so bone-crushingly tired today – and not all of them have to do with sleep.…