Check the details carefully about eligibility for the tour, hours, and whether you can take it with your flight schedule and be deposited back airside, although if your flight is operated by SQ you can check in as much as 48 hours before the flight. I've checked in late in the evening before a morning SQ flight and gone to the lounge (in fact, TPR) for dinner before spending the night in the airside transit hotel and then returning to the lounge for a quick breakfast before the flight.

BTW the SQ lounge showers are more luxurious that those that you will find in your ensuite bathroom in the transit hotel.