lack of light will cause u alot of strech....i wouldnt just yet to say its cuz ur growing sativa.....hell i got a pure indica under great light....just as its 3 others sisters have but yet only one streched like that....plants are all different man...even if they are the same strain doesnt mean it will grow the EXACT same way even when grown under the same conditions...this will prove my point...no pix of it..will get some to u soon tho...but ill explain...i have 2 NL down in the basement...both of these plants from seed has been growin under the same growing conditions...yet one is kinda streched...bit bushy but not really b.c of the streching...on the other hand its sister that sits exactly right next to her is VERY short...and VERY bushy compared to the other...plants dont all grow the same..all u can do is give good light and good growing medium....