This is a laptop hardware issue, not an ubuntu issue. I will explain. Generally speaking, Lenovos are the most well supported of all brands of computers when it comes to all distributions of Linux, so I would be very surprised if it were say, a matter of a driver.
Now if you open, this link, you will find that quite a few people share your exact complaint, practically word for word. When I did this search, I even found one hit, the fourth I believe, which indicated it had been solved, though I admit having opened that hit I can't claim I'm clear about the solution.
Either way, it looks as though the problem may be due to an ACPI device driver, cmos setting, or even bios flaw which can be solved by means of an update to your bios. If I were you I'd carefully inspect my acpi settings in my cmos settings. I might try taking the battery out and putting it back in.