9 ways I know of
All links are to PFSRD, the original sources are in brackets.
[Advanced Player’s Guide]
This cleric or oracle spell (3rd level) is extremely potent for sleep reduction as you can also get additional benefits in the same day for more sleep.
You reduce the amount of uninterrupted sleep or rest creatures within the spell’s area need ... to 2 hours instead of the normal eight ... every additional 2 hours counts as a day of rest
[Pathfinder 2: The Skinsaw Murders]
This prestige class gives you access to Efficient Sleep at 2nd level and isn't too difficult to pick up (but will require some less than perfect feats for a wizard).
at 2nd level, a spherewalker ... only needs 4 hours of sleep
[Advanced Class Guide]
This armor is just as good as the ring of sustenance for sleep reduction AND you get the added benefit of not becoming fatigued for sleeping in armor.
reduce the amount of uninterrupted sleep or rest she needs from 8 hours to 2 hours, and the wearer does not become fatigued by sleeping in this armor.
[Advanced Player's Guide]
Multiclass into sorcerer and acquire 3 levels of the Verdant Sorcerer bloodline to receive the Photosynthesis power.
At 3rd level, you feed upon nature’s raw essence. Your need to eat and sleep is reduced as if wearing a ring of sustenance
Multiclass into this unchained class and pick up Ki Metabolism at level 4 for your Ki Power option.
As long as he has at least 1 point remaining in his ki pool, the monk... needs only 2 hours of sleep each night
Grow a few of these trees near your wizards tower and learn to make some tea.
When a creature consumes a piece of the bark, it reduces the amount of sleep it needs that night by 2 hours (to a minimum of 6 hours)... A piece of bark brewed into a tea reduces the amount of sleep the imbiber needs by 4 hours (to a minimum of 4 hours).
[Heroes of the Wild]
An herbalist can brew up some tea from this flower which reduces the amount of sleep up to six people need to 6 hours.
this tea grants the drinker the benefits of a full 8 hours of uninterrupted sleep in only 6 hours. One dose of dreamer’s star makes enough tea to serve six.
[Pathfinder Player Companion: Blood of the Elements]
This regional trait allows you to...
sleep only 4 hours to get a full night’s rest (restoring 1 hit point per character level).
Awakened from Stasis
[Pathfinder Player Companion: People of the Stars]
Another regional trait (this one with a much more specific situation) . It's more effective than light sleeper but comes with a side effect of memory loss.
As a side effect of your stasis, you gain all the benefits of 8 hours of sleep in only 2 hours.
There is one more way but it requires a bit of luck and/or DM assistance:
A friendly druid, shaman, or witch can kill and reincarnate you until you are either Duergar with the Waking Dreamer alternative racial trait or Elan [both in Psionics Unleashed] each of which meditate for 4 hours instead of sleep for 8 hours.
You do not need to sleep at all and can meditate, fully awake and aware, for 4 hours to gain all the benefits of sleep