Very frequently all my life, whether sleeping alone or with someone else. Not every day but just about.
Mahatma Ghandi thought he'd gone beyond all sexual desire, because he purposefully hadn't had sex for many years and never woke up with a hard-on. Then one day, when he was quite old, he woke up with an erection, and that really upset him - he felt he wasn't as 'pure' as he thought he was.
"The Joy of Sex" said that if you wake up with an erection, use it!